i was in that electrum wallet (the one with the seed) and i clicked "new/restore wallet", then i clicked "import btc address or private keys" then i pasted private-key of the paperwallet.
Since the result was a "watch only" wallet then what you pasted was not a private key but instead it was an address. A private key should start with letter L or K.
And if this is what you did then you don't need to import any xprv or seed phrases, you just need to go to your paper wallet, find your "private key" (that starts with L or K) then use than in the same window as before in your new/restore section.