The TextApp number is associated with an active subscription (and so your Apple ID).

Please be sure that you have an active subscription and is not interrupted in recent times.

Likewise, be certain that you have not changed Apple ID - for example, originally subscribed through an alternate Apple ID than the Apple ID you are currently using to Restore the TextApp subscription.

In case you have verified all above possibilities and still, can't restore your number, please go through the guidelines and report the issue with details to get support.