Cause #1
You will not be able to enter into a period if it is locked or closed (check using the Open/Close period command in the Command menu).
Solution
If the period is locked, you need merely unlock it.
If the period is closed, you will not be able to open it again, and hence will not be able to enter the transaction into that period (after all, that is why periods can be closed).
Cause #2
In version 2 of MoneyWorks and CashWorks, you could not enter into a period if the Display filter in the Transaction List window is set to not display that period. This restriction was removed in version 3.
Solution
Change the Display filter so that it includes the period or the transaction you want to enter.