Unfortunately there's nothing much you can do about this. 
You'll need to log a request with the care crew here and they'll be able to look into this for you.

Have you reported your phone/SIM lost and ordered a new SIM?  

If you have then it prevents you buying anything until your new SIM gets activated.  Wait for the new SIM to arrive and you can then buy your membership.

Has it been a long time since you used your phone?  If you have not made any calls or texts for quite a while your number could be suspended or de-activated.  Contact the care crew here and they can check for you.

Have you ordered a replacement SIM?
If you have, then you'll just need to sit tight until it arrives, you won't be able to register until then.

If you haven't ordered a replacement SIM and you're getting this message then you can contact the care crew here to see what the problem is. 

Have you requested to move over your number to a new SIM?
If you're getting this error message then your transfer must still be in progress, just sit tight and this should be sorted for you soon.
If your number has already been moved then it's best to get on to the care crew here 

Has your number been transfered over yet? If not then just give it another little while and it should sort itself out. 
If your number has been trasfered over and this message is still coming up for you, then it's best to get in touch with the care crew here

Has your SIM been activated yet? If not then just give it a little time and it should sort itself out. 
If your SIM has been activated and this message is still coming up for you, then it's best to get in touch with the care crew here