Your subscription bought via App Store may be unavailable, when Termius doesn't receive the transaction receipt after the purchase, or you've made the purchase using Apple ID that is different from the one you used to install the app.
If you downloaded and purchased Termius using the same Apple ID:
Make sure you use the latest version of Termius.
Start Termius and navigate to the settings.
Under the Profile section, tap Restore Subscription to fetch the necessary data from iOS. At this point the problem should be gone, but if not, then proceed to the next step.
Navigate to your phone (iOS) settings and then to the iTunes & App Store section.
Sign out from your Apple ID.
Repeat restoring subscription, as described in steps 1-4. Enter your Apple ID, when prompted.
If you downloaded and purchased Termius using different Apple IDs:
Start Termius and go to the settings.
Under the Profile section, tap your e-mail.
Under the My Data section, tap Restore Subscription.
If none of this worked, please, raise a support request.