This indicates that your user account does not have the relevant period posting rights to allocate to the year and period of the transaction or transactions in question.
Remember that if your system is set to allocate to the latest year/period of selected transactions, the system will check the year and period on the transactions you have selected, and allocation will take place in the furthest forward period and year out of the selected transactions (so if you select two transactions from current year and one from next year, allocation will take place in next year).
To check if your system is set to allocate to the latest year/period, go to:
System
Options
System Options
Options
Transactions
You'll see the Method of Allocation option. If the system is defaulting to a future period, de-selecting this option should resolve the issue.
This can also be resolved by ensuring that your user account has the relevant future period rights to post into the relevant year and period.
Once your user account has permission to post into the relevant year and period, you will then be able to complete your allocation.