The filter cannot complete because of an unknown error

[Henry Solis]

In the latest version of Illustrator 26.3.1. I am trying to save a variable library and keep getting this error message "The filter cannot complete because of an unknown error" 

I am wondering if anyone else has experienced this issue.

When I revert to the older Illustrator version 26.2.1 variable library saves fine no issues.

[Anna Lander]

I'm not sure about this particular issue but the first thing to try is resetting Preferences. Use the Preferences button, or Edit > Preferences > General, or Ctrl/Cmd+K to open the General option in Preferences, click Reset and Restart Illustrator. If this does not help, well then we'll have to search for something else.

[Ton Frederiks]

If something still works in a previous version I would contact Adobe support:

Welcome to Adobe Customer Care! Need help?

[MikeGondek]

I imported a .csv file and  was able to Save Variable LIbrary to .xml on 

Mqc OS 10.15/7 and the latest Ilustrator 26.3.1

Can you try with a simple data file, might be an nonfriendly character in the data like a slash or colon. Just open a.csv in TextEdit and strip out all but a few fields. We will then know if this works on a simple file, or not at all.

[Ton Frederiks]

But I agree with Anna, resetting Preferences and restarting your machine is the first thing to try.

https://helpx.adobe.com/illustrator/using/setting-preferences.html