how do u export seed phrase
Log in to your Blockchain.com wallet, click on "Security Center" at the top right of the window, then click on the blue "Backup Funds" button near the bottom right, then again click on the new "Backup Funds" button. It will show you 4 words at a time, which you should write down on paper (
never store these electronically, copy and paste them to a text file, take a photo of them, upload them to an email account or cloud storage, etc. Paper only.) You can then use that seed phrase to import your addresses to another wallet.
However, none of this will help in your case. The non-spendable address in your blockchain.com wallet isn't covered by your seed phrase. As I said above, the non-spendable address has been manually imported as an address only, either by your or by somebody else. The only way you will ever be able to access these coins is if you have the private key from elsewhere - there is no way to recover the private key which relates to that address from your blockchain.com wallet.
You need to answer the following questions:
Where did the non-spendable address come from? Did you create it in another wallet, or did somebody give it to you?
Did you import the address yourself?
Did you create this wallet from scratch, or was it given or sold to you?