CRM 2016 on premises plugin - 'Microsoft.Xrm.Client' not found

Hi

I have a custom workflow step (Snadbox) plugin in which I want to clone related records of the primary workflow entity,

but I get the error below when I run it.

Could not load file or assembly 'Microsoft.Xrm.Client, Version=7.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' or one of its dependencies. The system cannot find the file specified

Changed from Sandbox mode to None but error still present.

Thank you