I'd first check if you have an access role that allows you to run the activity in the given class. Make sure you have a valid access role to object instance. Also, check if there is any access deny rule that may block this operation.
If this is an ootb activity, normally SysAdmin4 role would allow it. Please double check.
Could you please check the below settings for your activity.
To running this activity standalone, its always recommended to check the "Allow direct invocation" checkbox and UNCHECK "Require authentication to run" checkbox. Please perform the above steps and retest the scenario.
If even after performing the above steps you are facing the mentioned issue, then please test your scenario with any OOTB activity. Make sure to have SysAdmin4 role for the operator.