There are a couple of reasons why you can't assign a FOB.

There may be uncompleted transactions on the vehicle.

There may be another vehicle still assigned to the FOB

Clear Uncompleted Transactions

The first thing to do is check if there are uncomplete transactions on the vehicle.

• Open the Vehicle Transactions screen.
• Select All from the date drop-down box.
• Type the registration number into the Registration box.
• Check the box for Only Search Uncompleted Vehicle Transactions.
• Click on Search. This will show any uncompleted transactions for the vehicle.

You can manually complete the transaction. 

• Click once on the row in the grid to select it.
• Click on the Manual Complete button.
• This will open the Need To Know window.
• Select a reason why you are completing this transaction. Usually you can select No reason.
• Click on the Complete button.

Repeat if necessary to complete all uncompleted transactions on this vehicle.

You should now have no uncompleted transaction left on this vehicle.

This is a good opportunity to check whether the FOB is assigned to a vehicle.

• While in the Vehicle Transactions screen, insert the FOB into the FOB reader.
• Click the Get Vehicle by FOB button. If vehicle details are displayed in the Registration or Stock boxes, then a vehicle is assigned to this FOB and needs to be cleared.
• Make a note of the registration or stock ID.
• Click the Back button to go back to Main window.

Clear a FOB

• Click on the Vehicles button to open the Vehicle screen.
• Enter the registration number in the Registration box.
• Click the Search button.
• Click once on the row to select it.
• Click the Edit Button.

In the  Edit Vehicle screen click the Clear Number button. This un-assigns the FOB from the vehicle.

Click on the Save button.

You should now be able to assign the FOB as you wish.