Unable to make changes in my existing CDT

I was trying to add one field to my existing CDT and as soon I add I am getting the following error message.

HTTP Code: 500
APNX-1-4198-000

Error Evaluating UI Expression
Expression evaluation error [evaluation ID = 7UYV2XAP] : [valid:false,syntaxError:false,value:Expression evaluation error: An error occurred while executing a save: Expression evaluation error at function fn!objectReadAction_appian_internal [line 8]: blocked by: [FORBIDDEN/12/index read-only / allow delete (api)];]