Assembly must be registered in isolation error while trying to register plugin \ custom workflow activity in Isolation Mode None in Dynamics 365 On Premise

While trying to register our custom workflow activity from Isolation mode – Sandboxed to None for Dynamics 365 On Premise we got the below error

This was because the user through which we were trying to register the plugin in None Isolation Mode using Plugin Registration Tool was not having Deployment Administrator role.

Adding the user to that Deployment Administrator role through Deployment Manager fixed the issue.

https://docs.microsoft.com/en-us/dynamics365/customer-engagement/on-premises/deployment-administrators

Read more about it

https://docs.microsoft.com/en-us/previous-versions/dynamicscrm-2016/developers-guide/gg309620(v=crm.8)#security-restrictions

Hope it helps..