If you come across a message that Google Sheets can't save your sheets and you need to revert to an earlier version, but even the earlier version can't save, don't bother doing any of these suggested steps. Clearing cookies won't help. Disabling extensions won't help.
Instead, try this (as counterintuitive as it seems): copy and paste your Google Sheets cells into a new Excel workbook. Save the Excel workbook and upload it to Google Drive. Open it with Google Sheets. Save that sheet. Delete your old, non-working sheet.