Commands for "Nextcloud Mail"

How do I have to implement this so that it also works on the Syncloud server?

"Nextcloud mails are not displayed - solved
What has now helped is to delete the account from the database.
If you only have one account, then it is ID=1 in the database. I
deleted it using the MySQL command, but you can also delete it directly in the table using phpMyAdmin:
DELETE FORM oc_mail_accounts WHERE id=1

Not aure what it does but if you want to execute sql on syncloud, there is an sql shell to use:

Be careful as you can break nextcloud with sql commands.

When I execute this SSH command, I get an error message saying that it does not recognize this command.
What can I do?

Show me the screenshot please.

The link above explains how to open sql shell by executing this command:

snap run nextcloud.psql

Then you can execute sql commands.