When I have gotten this error message in the past, it was usually because of syntax errors in the feature code. So, as a sensible first step, check your braces and semicolons.

Thanks, oldnick. It's in order as far as I can tell:
@lining_fig = [zero one two three four five six seven eight nine];
@oldstyle_num = [zero.onum one.onum two.onum three.onum four.onum five.onum six.onum seven.onum eight.onum nine.onum];
@lowercase = [a b c d e f g h i j k l m n o p q r s t u v w x y z];
@small_cap = [a.smcp b.smcp c.smcp d.smcp e.smcp f.smcp g.smcp h.smcp i.smcp j.smcp k.smcp l.smcp m.smcp n.smcp o.smcp p.smcp q.smcp r.smcp s.smcp t.smcp u.smcp v.smcp w.smcp x.smcp y.smcp z.smcp];

When I exported earlier though, I deleted all this code and I still got that error message.

Here is my very unsophisticated take on the subject:

I have found that particular FontLab error message to be quite frustrating, not the least because the message is very easy to miss. Here are a few of the things that have sometimes gotten around it for me:

Making sure that all the name fields in FontInfo are filled in
Shortening the name(s)
Ensuring that the encoding fields in FontInfo aren't blank
Generating the font as a TTF and then generating it as OTF (the software equivalent of kicking it)

Thanks for the input, HVB. I'm not clear on where the name or encoding fields in FontInfo are. I generated the font as a TTF with no problems, but I encountered the problems described when I generated it as an OTF.
Right now, I'm downloading the Adobe Font Development Kit – maybe it has some helpful troubleshooting tools? Thanks again.

You get to FontInfo by clicking on the upper-left-most icon in the main font window - it's right under the 'F' in the font window's title bar.

Thanks for this, HVB. All the name information appears to be correctly filled in – the only section that is empty, is "Additional OpenType names."

I am considering creating a new FontLab file and then copying over all the glyphs from the current one I am working on. Is this a bad idea?

Negative: it would be time consuming. Positive: it might save time in the long run because I'm working from someone else's file, which means I might never find out where the problem lies. Also, it would help me understand FontLab by working from the ground up.

I've downloaded Adobe FDK and although I am new to it, I have yet to see find something in it that deals with my issue. Pretty lost here. Any help, direction, and/or thoughts would be appreciated. Thank you.

Does Fontographer open your .vfb file?

Working in FontLab – don't have Fontographer. FontLab does open the .vfb file though.

"NEW! Reads and exports the FontLab VFB format for easy integration into a FontLab-based workflow"
http://www.fontlab.com/font-editor/fontographer/
and
"All fonts created by the demo are 100% valid and can be tested in any application."

Thanks for the reply, JanekZ. I am not ready to purchase Fontographer at the moment, but that sounds like a good option for the future. Thanks again.

This error seems to be the catch-all when something interrupts the OTF file creation (regardless of whether the problem is actually related to the features).

Some things to check:
Do you have any glyphs which are referenced in the Opentype features or classes which are empty (no character or composite in the glyph)?

Are they any open contours anywhere? I've had a few instances where a single, unconnected node point showed up in a random glyph.

JamesT,

Thanks for the reply. I checked and don't have any empty referenced glyphs. Will double check. Will also check for open contours.

I noticed that when the font fails to generate, FontLab creates a file called ".notdef" which appears to be a blank glyph. Not sure if this is a "clue."

Today, I made a new font file, imported the referenced glyphs (lining figures, old style numerals, lowercase letters, and small capitals) with their respective feature code. Attempted to generate a new Opentype PS font with this. Did not work.

Try repeating your import process, but with only a few glyphs at a time. That should isolate the problem to a specific subset of glyphs.

Two other options to consider when indeterminate voodoo is afoot in the FontLab file: a) Generate a Type 1 version, then open it up in FontLab to see if the problem disappears; or 2) Generate a TTF font and check to see that all of the characters are present (in Windows Font Viewer—double-click on font file—or FontBook).

The latter option will show if there is a particular character that is causing problems: I had a case where none of the characters after the zero showed up. I rebuilt the zero from scratch, and everything worked fine after that.

May or may not help; can't hurt…

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.notdef is the character that is used to represent a character that is not encoded in the font you are using (if, for example, you attempted to type a Cyrillic character in a font that didn't support Cyrillic), it is normally a question mark or an x enclosed in a rectangle.

I don't know if excluding this character would cause any issues with generating the .OTF.

Thank you again for all the feedback everyone.

J.Montalbano – removed all the underscores, so the feature definitions are each one word (@liningfig, @oldstylenum, @lowercase, and @smallcap). Did not fix problem, but I will steer clear of them unless I hear otherwise from now on.

oldnick – generated a TrueType version and characters were all still there. Tried a Type 1 version, but not sure what I was looking at.

JamesT – thanks for the info on the .notdef character. Very good to know.

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Yes. J.Montalbano solved it. Thank you very much.