Looks like that datatype is using Contentment.
That’s a community package.
Based on what you’re saying - you’re trying to upgrade to Umbraco v15 and then hit this error?
After upgrading check nuget for a later version of Contentment. I’m not speaking from any experience here but I suspect you need a later version for v15.
There’s a thread here that might be relevant talking about issues with v15 / v16 Umbraco and contentment.
Lee K might be your man to advise here.