I am very familiar with the ME51N transaction and we're also creating flavors for it. Please don't quote me as an expert but what I think could help resolve your issue is that you must populate the Account Assignment Category field in Basic View, or the flavor even I guess, and proced with personifying it. So, you personified the PR in the state of a K, now you should save those changes and do it again for C. Don't hide any tabs until you've grabbed all of the fields you need. There are certain fields that only appear based on what you populate.
In my experience, I had to do this twice for Item Category blank (materials) and D Services. Each instance required that I simulate each PR and then personify the flavor. Hope this makes sense and helps.