Sorry that the Lightroom premium subscription is not getting activated.
Could you please try the below solutions?
Sign out from Lightroom CC mobile app and sign in again.
Now, check if the issue persists. If the issue isn't resolved, try Solution 2 below.
Please go into Lr> Tap on Upgrade Now opened from the search screen. This will re-initiate the purchase workflow.
Re-check the Gmail id and the Adobe id are correctly typed in for activation.
Open up iOS setting > iTunes&App Store, then long press on your Apple-id and choose "View Apple ID". Navigate to the "Subscription" entry. Check if the Lr Premium Subscription status is marked as 'Expired'?
Sign-out via iOS settings->iTunes&appstore->then start the Lr purchase process again. Once prompted, sign-in with the Apple-id and finish the process with final OK confirmation.
In addition run a "Restore Purchases" from the Lr Mobile settings.