Sharepoint - Can't Activate Solution from Site Settings in SharePoint foundation 2010
Can you check that ou have enabled and configured the user code service on your farm and started it?
Look for user code service in service applications and create one if it is not there.
Then go to
Central Administration > System Settings > Manage Services on Server > check that “Microsoft SharePoint Foundation Sandboxed Code Service” service is running
I solved this in SharePoint(2013) Central Administration/Application Management/Manage Services on Server by starting the Microsoft SharePoint Foundation Sandboxed Code Service