how do u export seed phrase
There is no way to directly access the private keys for an address which Blockchain.com created. If you want to see the private key, you have to export the seed phrase to a wallet like Electrum, and then use it to extract the private key. Blockchain.com will only show private keys which have been manually imported to the wallet.

The only way OP can have a "non-spendable" address in his blockchain.com wallet is if either he or someone else manually imported the address. OP will need to answer your other questions before we can go any further with this.