Since you didn't provide more information, I am assuming the following :
First, ensure that Visual Studio, SSDT, and any relevant extensions or plugins are fully up-to-date. Sometimes, issues like this are addressed in minor updates or patches.
Since you've mentioned that Tabular Editor works fine, you may want to use it as a temporary workaround for managing the Ordinal property of your calculated items.
Sometimes, errors like this can be misleading. Double-check that the syntax and constraints surrounding the calculated items and the Ordinal property are correctly set up or try recreating the calculated items from scratch to see if the issue persists. There may be an underlying corruption or misconfiguration that's causing the issue.