A custom role with Purchase Order permission set to Edit access level is unable to edit Purchase Orders via Inline Editing. Even after completing the conditions set in SuiteAnswers ID: 11844.
Check if the role has the Restrict Time and Expenses set to it.
1. Navigate to Setup > Users/Roles > Manage Roles.
2. Edit the custom role.
3. Look for the Restrict Time and Expenses checkbox and unmark it.
4. Click Save.