Is the email address the used to set you up as a new user, already being used as a username for your personal account? If so, you'd need to login to your personal account, andupdate their username to something different / unique. Once you've done that you should be able to access the coded link in that user invite email.
It'd also be advisable that once you've gotten your login setup for the client's account, that you update your username for that to something more unique - that way you're free to have your email address used to setup another account user login if necessary.
If you need further assistance with updating your personal accounts' username, or getting setup as a user for your client's account, I'd advise calling general supportto provide the email address in question, confirm security permissions live, and work with the agent to administratively update the info.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ William A Community & Social Media Support