If you deleted a product directly from Amazon Seller Central but its ASIN still appears linked inside the app, the app simply hasn't received Amazon's deletion notification yet. The link clears itself within 7 days, or you can speed it up manually.
If you don't want to wait the full week, manually remove the product from Listing Not Found before the 7-day window closes — the linking is unlinked immediately.
If you upload the same product to Amazon again while the app is still cleaning up the old record, you may see processing delays. Wait for the unlink to complete (or trigger the manual removal) before re-uploading.