In the allocation window it is likely you have the Currency set to All (in red). You can only use the All setting when allocating accounts set to any currency if the selection of transactions you are allocating are of different currencies.
If the transactions you are allocating are all the same currency (as indicated by this message) you will need to change the currency on the allocation as follows:
With the allocation screen open, select the button marked All
Select the correct currency code from the drop down
Once you have set this you will be able to allocate the currency values.