To make non-spendable Bitcoin become spendable, you'll need to have access to the private key associated with the Bitcoin address in question. If you don't have the private key, then the Bitcoin is indeed non-spendable. You can't deduce a private key from a public key or address, so if you didn't receive the private key when the address was created, it's unlikely you'll be able to spend the Bitcoin.
However, if you do have the private key, you can import it into a wallet that you control, such as Electrum or Bitcoin Core. Once the private key is imported, the Bitcoin associated with that address will become spendable. It's important to note that if the Bitcoin address in question is a watch-only address (i.e., you can see incoming transactions but can't spend the Bitcoin), then it's likely that you don't have the private key and therefore can't spend the Bitcoin.
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If you're dealing with a non-spendable Bitcoin balance on a platform like Blockchain.com, it's possible that you've been the victim of a scam. In this case, it's unlikely that you'll be able to recover the non-spendable Bitcoin unless you have the private key associated with the address. If you're unsure about the status of your Bitcoin or the private key, it's best to consult with a trusted Bitcoin expert or use a reputable wallet or exchange.