The register for period in Palette is found under Administration > Register > Periods. To see whether the period is open, search for the relevant period and look in the column Closed.
To open the period, you must know if your integration between ERP and Palette supports transfering of Periods. If it does, you should make the change to your period in the ERP system. The change should then be transfered to Palette.
If you manage periods in Palette only, click on the relevant period and make sure the parameter Closed is not checked. Dont forget to Save. You should now be able to transfer the invoice from the invoicelog.