If you read the release notes for 0.21.0, you'll see the descriptor wallets are regarded as "experimental" and should not be the "default" wallet type created:
https://bitcoincore.org/en/releases/0.21.0/Additionally, the import and export functionality for these types of wallets is different:
As Legacy Wallets and Descriptor Wallets use different mechanisms for storing and importing scripts and keys the existing import RPCs have been disabled for descriptor wallets. New export RPCs for Descriptor Wallets have not yet been added.
The following RPCs are disabled for Descriptor Wallets:
- importprivkey
- importpubkey
- importaddress
- importwallet
- dumpprivkey
- dumpwallet
- importmulti
- addmultisigaddress
- sethdseed
So, as you can see... there are no "Export" functions for descriptor wallets as yet. So you will not be able to export anything at this time.
Note that v0.21.1 is just the update to support Taproot... so the 0.21.0 info regarding descriptor wallets should still hold true.
Essentially, unless you explicitly need descriptor support (and you can only use it for importing at this time), I would suggest that you continue to use the "legacy" wallet option by making sure that the "descriptor wallet" option is not used during wallet creation.