Unable with Gmail Connected system

HI,
 while connecting with Gmail connected system using service account , facing error : 

HTTP Code: 500 APNX-1-4198-000 Error Evaluating UI Expression Expression evaluation error [evaluation ID = adefc:4d51e] : [valid:false,syntaxError:false,value:Expression evaluation error: An error occurred while executing a save: Expression evaluation error at function a!connectedsystems_testConnection_appian_internal [line 51]: ]

Please let me know If anyone have solution.