The system will only allow you to add employees as long at they are in a post and have pay details. It is not possible to add an employee with no payroll details but you can either add them on a zero hour contract or create a user account for them, depending on the case.
Add an employee on a zero hour contract
If you need to create an employee with no payroll details, they will need to be added on a zero hour contract. You can then set them up to be paid for payroll claims and they will not receive any payments unless payroll claims are created, approved and exported for them.
They will still need to be added in a post and will receive an employee number (payroll number) as well.
Create a user account
The other option would be setting up a user account.
This is not recommended for regular employees and is mostly used for admin users who need full access to the system in order to make admin changes to other employees.
Users do not need to be added in a post, unlike employees on a zero hour contract.