J
Try baking the constraint into the table header, and have an interaction when a user changes the 'X' per type.
I don't know how frequent a user will be configuring this, so you could use the table header to explain the constraints (the what); if more explanation is needed, use the info tooltip to tell them the why.
You can pair that with some feedback. When they select the dropdown to YES, a label appears, telling them what type this is the X for. This way the column sort can show them the YES's in a row, plus labels for each type represented, which is helpful if you have a decent amount of types.
Scenario: each type must have an x before proceeding
If selecting an X for every type is mandatory, you can pop a warning message above the table that tells them they must select an X for Type C.
Testing: try a version with proximity for Type and X
If you have many columns, one thing you can try is to place the related columns next to each other, to cut down on the scanning.