When you have created for an example SharePoint 2010 Event Receiver as a Sandbox solution using Visual Studio 2010 and when you are going to debug your project, sometimes you might get this error "Process 'SPUCWorkerProcess.exe' is not running."
The reason behind this error is, if you are using SharePoint 2010 on a Domain Controller then by default Sandbox solutions are disabled on it. So if you're running SharePoint 2010 on a DC you will need to add a ACL Access Rule to enable Sandbox Solutions. ACL(Access Control List ) means with respect to a computer file system, a list of permissions attached to an object. So without this Access Rule the Microsoft SharePoint Foundation User Code Service will start but the SPUCWorkerProcess.exe won't. Because by default it is disabled. To enable it, you will need to run the following PowerShell script. Make sure to run PowerShell as Administrator.
$acl = Get-Acl HKLM:\System\CurrentControlSet\Control\ComputerName
$person = [System.Security.Principal.NTAccount]"Users"
$access = [System.Security.AccessControl.RegistryRights]::FullControl
$inheritance = [System.Security.AccessControl.InheritanceFlags]"ContainerInherit, ObjectInherit"
$propagation = [System.Security.AccessControl.PropagationFlags]::None
$type = [System.Security.AccessControl.AccessControlType]::Allow
$rule = New-Object System.Security.AccessControl.RegistryAccessRule($person, $access, $inheritance, $propagation, $type)
Set-Acl HKLM:\System\CurrentControlSet\Control\ComputerName $acl