My BSQ were not sent

My transaction of selling BSQ by BTC is stopped.

I followed the procedures for the transfer of BSQ but they are not on the network.

The counterparty confirms that it has not received the BSQ.

What should I do? What could have happened?

Selling BSQ is like sending BTC. You need to do it through Bisq using the BSQ wallet at DAO > BSQ wallet > Send.
To know if a BSQ transaction has been sent, you should go to BSQ wallet > Transactions and by clicking the transaction ID you can see at a mempool - Bitcoin Explorer if the transaction is confirmed.

If you’re completely sure that you made the transaction but it’s not displayed, try to do a SPV resync but this transactions are usually sent fine and the most probable is that you just forgot a step to send.
It also makes sense to mention that you should check if your DAO synced. You can check at DAO > Network Monitor.

Also, If you have set up a password for your Bisq wallet you now need to enter this when sending BSQ. This was added in the recent update.