SAP LSO: Reference Document $$ could not be found

Dear gurus,

We are implementing SAP LSO. We wish to capture the prices of each course and hence have set up the activity allocation to CO.

(We don't actually need to integrate to CO, but it seems we must set up acty allocation in order to capture the prices).

However, I encounter the error message "Reference document xxxxx could not be found" when I try to change the price for an employee's booking.

Anyone know how to fix this? There is an old SAP note 448754 but we are on ECC6 EHP5.

Appreciate your help!

Thanks

Jessica