My sub-user can't see create invoice on the PO page.

Your sub-users will not be able to create invoices until they receive the necessary permissions by account admin.

Add permissions by following these steps:

1. In the upper-right corner of the application, click [user initials] > Settings > Users
2. Click Manage Users
3. Check the Role Name under Role assigned
4. Click Manage Roles > Role name
5. Tick Show me all the available permissions
6. Tick the Inbox and Order Access and Invoice Generation on page 2, and Outbox Access on Page 3.
7. Click Save.

Please also confirm the SAP Business Network Discovery Contact role is not enabled for the user by following the steps below:

1. Under the Manage Users tab, click Actions > Edit for the desired user.
2. Unselect This user is the SAP Business Network Discovery Contact, click Save.

Alternatively, you can create a new role containing the necessary permissions, or add to an existing role with the necessary permissions to your user's account.

SAP Business Network for Procurement & Supply Chain > Administration (Buyer & Supplier Account Administration)
SAP Business Network for Procurement & Supply Chain > Transaction Documents > Transaction (Invoicing)