The date that you enter in the GL period field in the Invoice entry screen needs to be in an opened AP period.
If not opened / "future open", then you would see the below popup error (App-sqlap-10006).
Do the following to open/"future open" the AP period in Oracle payables module,
Login to oracle>Payables super user responsibility>Setup > Calender > Accounting > Accounting periods
You would see the below screen.
Open the required period by selecting "Open"/"Future" in the Period status field and save the change.
Now, invoice can be entered in the open/future period.