There are 4 types of access rights within an account with Link My Books.
If you are unable to add a new user then you are likely to be a Basic or Accountant user.
You will need to ask the Owner of the account or a user with Admin right to add additional users to the account.
To find out more about access rights please read our help article How to add additional users to your account
If you have any questions regarding this please open a conversation with support via the blue chat icon on the bottom right of any page.