Trying to save a new file I have been working on, and get "Cannot find file" when I save.

@wcn_dave sorry to hear that you are having. I can only imagine all this work and then you can't save. Honestly, I would be pullin my hair out or swearing at my screen! So, you have already tried 'save as PDF first, change colour mode first etc' but no avail!

How are you saving your files? To the Cloud or Computer?

First, of all, can you 'Select All' and 'Copy' and then it create a new document and paste it in. Re-name this file with NO spaces (only hyphens and underscores) and also avoid using special characters like slashes, backslashes, colons, or question marks in the file name. And try saving (Save As) to your computer as either AI, EPS or SVG? 

What version of Adobe Illustrator are you using? If it's 2023 or 2024 that should be fine. Don't bother using the Illustrator Beta. Sometimes updates often include bug fixes and improvements. Try to use the latest version of Illustrator if possible. 

Try that first....Normally I would say reset your preferences, but since we want to try and save your files versus losin all your work....do what ever you can. 

if that still fails....try 'Exporting' your file as a 'SVG'

Hopefully one of these works for you!