It could mean that the transaction is invalid.
Since your node is synced, that transaction must have been marked from "Unknown" to "Unconfirmed: not in mempool" after scanning the blockchain.
Note: "Not in mempool" mean that the transaction is neither in your node's mempool nor in the Blockchain.

You can use testmempoolaccept command to see if you can still send it to the nodes, follow this:

• In the transactions tab, right click on that transaction, then select "Copy raw transaction"
• Open Bitcoin core's console, "Window Console" and type (including the single/double quotation marks):
  

  testmempoolaccept '["PASTE_RAW_TXN_HERE"]'

• If it says "allowed": true,, then it can still be mined and since it's an inbound txn, you'll receive it after some confirmations.