You are not setup as a worker , while trying to access PO Related Form from Oracle Enterprise Asset Management.
Please note that this error is similar to 11i error "You are not setup as a employee". With the addition of expanded Contingent Worker functionality available in R12, the error message to validate employee access to the forms has been changed from ' You are not setup as a employee' to ' You must be a worker to access this page'.
To Fix the Issue , associate employee with the users who are getting this error message.
Security > user >
Query for the user and assign employee to the user by populating the field "Person" as shown below