SQL Agent doesn't have perms to run a PowerShell script
I had a similar problem when running powershell scripts from a sql server job.


  If you do not have remote PowerShell enabled on both servers and
  firewall access between the servers configured properly it will have
  issues.


Run a PowerShell script on a different server - from inside a SQL Server job

the way I have solved this problem is through a proxy account.

I have created a proxy account in sql server, 
that had all the relevat permissions to the remote server, folders and files that I needed and run the sql job as that account.

a proxy account in my case was a domain account, and I have granted this account the privileges on the remote server.

this might help with proxy:
How to get the AD account associated to a Proxy?

From this link below you can see:

powershell problem when running on a remote server - access is denied

As far as workarounds, there is this answer on SU. you can read the article in detail that the answer points to fully understand, but this is the main command to adjust permissions:


  If you decide you want to allow others, what you do is run the
  command: 


Set-PSSessionConfiguration -Name Microsoft.PowerShell -showSecurityDescriptorUI 



  Notice that this action could have a serious impact on your system so
  we ask you to confirm that you really want to do this.