This doesn't sound like a Microsoft Office Runtime issue. The fact that Excel asks you to save with a different name and then still fails usually points to either a damaged Office installation, an add-in, or a problem writing to the folder you're saving to.
A few things I'd try before considering a reinstall:
Start Excel in Safe Mode (Win + R, type excel /safe) and see if the file saves. If it does, one of your add-ins is probably causing the problem.
Create a brand-new blank workbook and try saving it. If that works, the issue may be with the original workbook rather than Excel itself.
Save the file to a different local folder (like Desktop or Documents) to rule out a permissions issue with the current location.
Run an Office Quick Repair from Control Panel > Programs and Features > Microsoft Office 2019 > Change. If that doesn't help, try the Online Repair option.
Also, don't let Microsoft support convince you that you need to buy Microsoft 365 just because Office 2019 isn't under mainstream support. Office 2019 is a perpetual license, and this type of save error is usually something that can be fixed without purchasing a newer version.
One question that would help narrow it down: does this happen with every Excel file, including a brand-new blank workbook, or only with one specific workbook?