I thought your idea of logging in to the guest account and then attempting to erase the free space was a capital idea. But alas, no joy.
After booting back to my main account, I then repaired permissions and, without quitting DU, immediately attempted to erase the free space. Bingo! Voilà! Success!
Always, my thanks.
27" i7 iMac (10.13.6), iPhone Xs Max (12.1)
Those who can make you believe absurdities can make you commit atrocities. ~Voltaire