If a position in your wallet is worth less than 2 dollars, you will not be able to sell it through the app. The blockchain fees to process the trade would cost more than the position is worth.
This can happen for a few reasons. A small amount may be left behind after selling a token, which is called dust. It can also happen when a token loses most of its value and your remaining holding drops below the threshold.
If you want to clean up small balances, you can hide them from your portfolio view using the "Hide dust" toggle in the app. This removes them from your visible positions without deleting them from your wallet. The tokens are still there on chain, just hidden from the display.
If you want to fully remove them from your wallet, you can export your private key and use an external wallet to burn or transfer them. This is optional and not necessary for normal use of the app.