If you have already allocated in a different period and un-allocated you may get this message when re-allocating in a different period. The year and periods would depend on what period you previously allocated and are trying to allocate in now.
The period of at least one of the prior allocations of the transactions being allocated (Year/Period) Exceeds the specified allocation period (Year/Period)
To remedy this:
Click System, Options, System Options
On the Transactions tab, to change the method of allocation, clear the Allocate to the latest year/Period of the transaction selected check box. You will now be able to allocate to any open period.
Once the allocation has been done it is advisable to select this option again to cut down on any issues with your allocations.