Nothing pops up in the allocation screen when I try to write off a TOC posting

When the TOC was posted it will not have been flagged as a Disbursement.

It will appear in the ledger totals as a negative unallocated receipt.

To write this off you will need to post a BRO to the ledger picking up the TOC.

Now post a BPN to the write off nominal, entering the matter details when posting.

The BRO and BPN will contra on the bank reconciliation.