Hi Kendra,
Via https://www.reddit.com/r/Office365/comments/1bm8tzu/cannot_save_word_document_as_doc_or_rtf_error/, let me know if the solution helps.
In the Office menus, under Open, there was a heading with a yellow triangle labeled "Files Needing Attention". Two super-old randomfiles were listed under that tab. After deleting both of those files, I went back to the original document AND IT SAVED AS A .DOC!
I’m eager to hear back and work together to find a solution.
#2:
https://learn.microsoft.com/en-au/answers/questions/5426762/how-to-resolve-the-save-failed-due-to-out-of-memor
Standard 3rd party link disclaimer:
There may be links to non-Microsoft websites that provide accurate and safe information. Watch out for ads on websites frequently classified as PUP (Potentially Unwanted Products). There is no need to buy paid products to fix your computers, as they sometimes do more harm than good.