If you can not find a trace where it has been marked for settlement and still want to remove the link, navigate to ‘Settle open transactions’ form for Customer/Vendor. Select the particular transaction, right click -> Record Info -> Show all fields and copy the Record-ID.
For next step, navigate to AOT using the table browser to directly access the data of SpecTrans table. The table SpecTrans contains all transactions marked for settlement. Find the record-id you previously copied against RefRecID column.
Delete the found SpecTrans record to remove the marking.
Below post will be helpful.
ref:
https://community.dynamics.com/ax/f/microsoft-dynamics-ax-forum/224912/this-transaction-has-been-marked-for-settlement-by-another-user