To set up an account in Collaborator, you must be sent the welcome email by your trading partner. You will then receive an email inviting you to sign off a plan once it's been sent by your trading partner. If you have not been sent the welcome email, but receive an email inviting you to sign off a plan, then you will not be able to log in to Collaborator as you do not have an account. Similarly if you have not followed the instructions on the welcome email correctly to set up your account and then try to sign off a plan, it will not let you log in.
To set up an account, contact your trading partner and ask them to send you a welcome email.