Problem making multisig PSBT on walletcreatefundedpsbt
Create a multisig with bitcoin-cli createmultisig 2 &quot;[&quot;pubkey1&quot;, &quot;pubkey2&quot;]&quot;

Here is your problem. createmultisig is a utility function that only computes the multisig address. It doesn't store or otherwise add that address to your wallet. As a result, when you invoke walletprocesspsbt to update/sign, the wallet has no idea what that address is, or how to sign for it.
You want to use addmultisigaddress instead.

To add the redeemscript it says is missing, can I use walletcreatefundedpsbt?

walletcreatefundedpsbt is an RPC to construct a PSBT from scratch; it's an alternative to the more manual createpsbt call, not a way to update an existing PSBT. You need to use walletprocesspsbt PSBT for that, but as I said, you first need to teach the wallet about the address you're using.