Excel Error "No more new fonts may be applied. Please close some other documents and try again"

ruibm suggestion to fix problems within the doc also worked for me - The spreadsheet had a few circular references and formulas that were bonked, File > Info > Check for Issues > Inspect Document located the issues and we repaired each. The ambiguous font error also disappeared when the formulas were corrected and updated. If you close the file in question and open it back up, it should present you with a message indicating the underlying problem with the document. Fix the errors, save the file and close it. Once the error message disappears upon reopening the file, you should be able to continue working on the file without seeing the font error again.