when im trying to upgrade a certain site collection I get this error

Prepare for takeoff!

Upgrade is currently disabled for your site collection. Please check back later to see if it has been unlocked for you.

tried

$site=Get-SPSite <URL>
$site.AllowSelfServiceUpgrade=True but PS came back with an error

True : The term 'True' is not recognized as the name of a cmdlet, function, script file, or operable program. Check
the spelling of the name, or if a path was included, verify that the path is correct and try again.
At line:1 char:31
+ $site.AllowSelfServiceUpgrade=True

any ideas ?

Thanks