I have fought for some time with the error below:
Non-translatable field elements can only be changed when updating the original language.
I have "inherited" a site a few years ago and been updating it so it's D8 up-to-date (8.9.16).
When the user tries to save a change in language version of a page (on many but not all pages), she gets the aforementioned error message (even if not changing anything on the form).
After researching the problem a bit I found out about the module: Remove Entity Untranslatable Field Validation. I installed it and it made its magic going through the pages but the problem just didn't go away after that.
Other than that there's not much help in the web and I am running out of options, does anyone here have any suggestions what to try next?