A couple other things.
1) some people forget they don't have write permission to certain directories. IF it still won't save using Save-As, then try saving to your Desktop instead and see if it saves there.
2) make sure you are saving the tax file and the PDF file to yoru computer, and not to some cloud drive.
3) DO not save directly to a flash drive from the TTX software. Historically, those files get corrupted in the save. Save to your hard drive, then copy those to the flash drive.
____________*Answers are correct to the best of my knowledge when posted, but should not be considered to be legal or official tax advice.*
