Upgrade is currently disabled for your site collection

You get the message that upgrade is currently disabled when you try to do site collection upgrade option under Site Collection administration or you try with Powershell.

Use the below command in powershell to enable the site for upgrading.

$site=Get-SPSite "http://SPSITE"
$site.AllowSelfServiceUpgrade=$true

How to deploy WSP Solution in compatibility mode using powershell

As there is no option in Central Administration to deploy a solution in Compatibilty mode, we can use powershell to deploy a solution.

The Suggested method for this type of deployment is:-

Install-SPSolution –Identity Solution.wsp –GACDeployment –CompatibilityLevel {14,15}