If HM Revenue and Customs (HMRC) hasn't been able to issue an authorisation code to your client, an email will have been sent to you to explain what to do. If the Online Agent Authorisation service shows that a code has been issued, the most likely reason is that the address HMRC holds for your client is wrong.
The authorisation request will also fail the validation process if an agent's address isn't held on HMRC records.
Your client should phone the tax office to check the address is correct. If changes are then made, please wait 72 hours before requesting another.