To tax the bike in this country you need a valid MOT certificate, insurance and the registration document for the bike - if you were missing any of these then you would not be able to get it. I suspect if you brought the bike over from Portugal then the registration document (proof of ownership in effect) might be the problem. How long have you been here? I am fairly certain that with foreign vehicles you are OK for six months but after that time you then need to register it with the DVLA and get a certificate and then you can get it taxed.