This may be somewhat obvious but took me a few minutes to figure out what had happened. I have been developing sandbox solutions on my development box for a project I'm working on and was at the point of uploading some of the WSP's to the staging SharePoint 2010 farm. They uploaded just fine to the site collection and showed the "Activate" button highlighted upon selecting the solution. Once the activate dialog box came up, however, the activate button was disabled as shown below:
The message states the control is disabled and it could be due to permissions, etc. My first thoughts were to double check that my permissions were setup correctly on the staging farm (and of course I went about checking that) but then realized that it the “SharePoint User Code Host” service was disabled which was of course due to the “Microsoft SharePoint Foundation Sandboxed Code Service” being Stopped (or probably in this case just never started to begin with).
Resolution is to simply go into Central Administration and navigate to System Settings –> Manage Services on Server and start the “Microsoft SharePoint Foundation Sandboxed Code Service” service.