When trying to apply a payment to an invoice in Dynamics 365 for Finance and Operations a person with a cross icon is displayed on the Settle transactions form. Typically, you could click on Inquiry> Specifications to see the transaction it is marked against. If this is blank you are not able to unmark the transaction to mark it again correctly.
To resolve this issue, you need to can now run the Consistency Check (System Administration> Periodic Tasks> Database> Consistency check) and selecting the Payments option. This allows users to unlock these transactions for them to be marked again to be settled. This consistency check will identify and clean up the orphaned settlement (SpecTrans) records.
For previous versions of AX you can find information on how to resolve this by following KB 894412: https://support.microsoft.com/en-us/help/894412/a-red-hand-symbol-appears-next-to-an-invoice-in-the-open-transaction-e
If you do not see the Payments option in the Consistency Check form please download KB 3187699 via LCS: https://fix.lcs.dynamics.com/Issue/Resolved?kb=3187699&bugId=3776531&qc=32c247d250343e3ddf31e7f5d74bff96793abd51d340836f4bc3608d95ac05a6