Hi,
It seems you had changed the field reference for the material type or the screen sequence. Please check the below points:
1. Go to OMS2 and check the field reference key assigned to the material type.
2. Go to OMT3E and check the screen sequence assigned to the combination of the screen reference of the material type. In standard, the screen sequence is 21. If its different, change it to 21 and test the scenario again.
Regards,
AKPT