This error usually suggests that the main currency is missing from FocalPoint Currency Rates and the Purchase Requests are looking for it, unable to find it and then the system errors.
To check this:
Log into FocalPoint Administrator
Click System Control, Currency Rates
Ensure that the home currency is listed in this window and that when you edit it, any other currencies you may use are listed with exchange rates. If any are missing, you will need to add these in by clicking in the detail line section and populating the relevant fields