If you would like to select a grade for an employee from the Grade/Rate dropdown when creating a claim for them:
Search for the employee and go to their profile.
Click Employment, then Position.
Click Post Details.
Click Edit in the Basic Details area.
Untick the Multiple Positions box.
📌 Note: By doing this, the Position dropdown will no longer be available for employees in this post, but the Grade/Rate dropdown will function correctly.