Hi Karen10,
You would need to log on as an administrator to have access to the User Access button under set-up. Or, failing the above, you could log into my.MYOB and look under Manage Users. If you don't have manage users under my.MYOB, the client may have changed your user access role, and that would be why you don't have the User Acess button.
Feel free to post again anytime you require further assistance.
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Kind regards,
Shella