Trying to upload calc template, "there were some errors"

HI,
trying to share a calc template, but keep getting a “there were some errors” message.
Everytime I fill out the data on one of the tabs and hit “save” I get the error, and the screenshot and source file gets removed.

Tried it with Chrome and Edge.

Any ideas whats wrong?

thanks,
baxtrom

Found out my mistake, wrong file extension (spreadsheet ods → template odt).