Ethereum can store whatever bytes you want, so you can certainly encrypt a string and store that. But everything on the blockchain is public, so there's no place to store a decryption key. Even if the decryption key is off-chain, if it's ever brought on-chain to do decryption there, then everyone can immediately see it.
So encryption is only relevant of decryption is done off-chain. For example, you could encrypt something with a recipient's public key, and off-chain, that recipient could use their private key to decrypt it.