Is it showing "In-app purchase are not allowed" while you are trying to activate the subscription on your IOS device? then you have to change your settings.
Follow the steps mentioned below to activate your subscription.
Go to settings and then click on Screen Time
Then click on Content and Privacy Restriction
Inside the content and Privacy restrictions, You can turn it on by clicking on the switch icon
You can ensure the settings is turned on if a green colour appears on the switch
Then click on iTunes & App Store Purchase
Then click on In-App Purchase
Then click on the Allow button and make sure a tick mark appears on it.
Now you have changed the settings and you may proceed with the payment for the subscription.