Hi there, mipson.
It's possible the email used by your client when inviting you as an accountant is the personal one. You can inform your client to delete you first as an accountant in their Online account.
Then, let your client invite you again using your company email address.
For more details about adding and removing accountant user, you can refer to this article: Invite or remove an accountant.
You can always reply to this post if you have more questions. I'm here to help.