After uploading this solution to the solution gallery, we need to activate the solution. If the Activate button is disabled, we may need to start the sandboxed service in the farm.

Go to Central Administration > System Settings > Manage Services on Server.

Check that “Microsoft SharePoint Foundation Sandboxed Code Service” service is running

This error may appear in some cases if there is something wrong with the sandboxed service.

Typically restarting the service should fix the problem, otherwise we can try to increase the thresholds.

default: 30

$uc = [Microsoft.SharePoint.Administration.SPUserCodeService]::Local

$uc.WorkerProcessExecutionTimeout = 5000

$uc.Update()

default: 1

$tier = $uc.Tiers[””]

$tier.MaximumWorkerProcesses = 2

$tier.Update()