There is probably a code problem. You can check by adding a new target language, e.g. Zulu: if you do a Pseudotranslation of that language (menu Translation) and can export that pseudotranslation, the file is exportable (Pseudotranslation inserts the codes correctly). If your language isn't, there is a code problem. (trick added by Walter Weyne)
Use the command Check embedded codes - Ctrl + Shift + F8. When DV stops at a row with a code problem a description is given (in small print!) below left in the status bar.
Checklist:
1. The target segment must match the following possibilities with the source segment:
a. source begins / does not begin with a code
b. source ends / does not end with a code
2. Codes must appear in the same relative order.
3. Codes that are adjacent in the source (no intervening characters) must appear that way in the target.
4. Codes that are separated by one or more characters in the source must be separated by one or more characters in the target (not necessarily the same characters). (You can add a character such as ~ or $$ to find that location in the exported document and correct it if necessary.)
5. The codes are bracketed by curly brackets, for example {0039}, with the code and curly brackets greyed out. Sometimes, though, a code will "lose" one of its curly brackets, and become attached to another code, for example [0039}0040}, with the entire unit greyed out and a stray { in regular black text somewhere in the section. The solution is to delete the entire unit, use F8 to reenter the correct codes, and then delete the stray black bracket. (DejaVu doesn't like "{" and "}" in the text.