Years ago I had put BTC into a store wallet. The store went bust and I didn't get to transfer my BTC into another wallet before I lost access to it. I can still see the balance on blockchain.info and I have the PIN code provided to me by the store to verify my ownership of the coin, but I don't know if I can access it, and if I can, how would I go about it?
I don't know if I can access it
I can still see the balance on blockchain.info
That means you probably know the Bitcoin-address associated with some amount of money. However, knowing an address doesn't give you any control over that money.
There's a common phrase in the Bitcoin world:
Not your keys: not your Bitcoin
The only important information stored in a Bitcoin wallet is a secret number known as a private key. It is knowing that private key that makes it possible to spend Bitcoin money. Without that key you have no control over the money.
An account with a store is almost always a custodial wallet meaning the store has custody of the keys and you don't. Your PIN just tells the store who you are. That PIN is now useless.
This is why many people recommend non-custodial wallets, not custodial wallets (accounts). Especially for long term storage of money that isn't being actively traded.
You need the private key or the seed-phrase (also known as a recovery phrase or backup phrase) from which the private key can be derived. These phrases are typically made of 12, 18 or 24 words from a standard list of about two thousand words. It seems you don't have this.