Activate button Disabled in SharePoint Sandboxed Solutions

Developers reported an issue in a New SharePoint 2010 Environment: When they try to activate sandboxed solutions “Activate” button is disabled (greyed out)! This is because: By default, sandboxed solutions are disabled in SharePoint.

Confirmed, SharePoint 2010 activate sandbox solution disabled. Confirmed user has enough permissions (User needs to be a site collection administrator or with Full Control permission at the root site collection, to deploy and activate sandboxed solution), logged in to the server and found “SharePoint 2010 User Code Host” is disabled from Services console.

Solution: 

Enable Sandboxed solution by starting “Microsoft SharePoint Foundation Sandboxed Code Service”!

Go to Central Administration >> System Settings >> Manage Services on Server, start “Microsoft SharePoint Foundation Sandboxed Code Service”.

Microsoft SharePoint Foundation Sandboxed Code Service can be stared with PowerShell:

Start-Service -Name SPUserCodeV4

Now, you will be able to upload & activate sandbox solutions!