Descriptor wallets do not support dumpwallet command.

Use listdescriptors false instead,
you can change the arg to "true" to show the "master private key" instead of the "extended public key" in case you need to derive the address' private key.

Also, I suggest you to create your own topic since there seem to be more of it than using that command.
Another note, do not entertain any PMs that you'll receive, keep the non-private conversation public as much as possible.

It will work with or without the .txt file extension.
The only difference will be the compatibility of the output with certain txt editor applications.