In 4.6.10 we made a change to how Forecasts can be created or refreshed.
The system will now only allow the creation or refreshing of a Forecast in the first open Forecast Period, which is the Period that displays when you first select the Forecast tab.
If you believe that you have selected the correct Period and are not able to create or refresh, then the Period Ends maintenance screen will need to be checked
After opening the Period Ends screen, change the filter to "Forecast Closed = no".
Then review the open Periods and close the previous/old periods that are not required.