Looks like 33 characters

That could be a bitcoin address. You cannot recover access to the Bitcoins at that address using only the address. To use those Bitcoins you need to know the private-key for that address.

It could also be a Wallet-ID. That would probably mean you have an online wallet where a third party (coinoutletatm.com in your case) controls your wallet. If so you should contact coinoutletatm.com to find out how you can use this Wallet-ID. You can probably use it at any coinoutletatm.com Bitcoin-ATM. You may need to also remember a password and/or use a 2-factor-authentication method established at the time.

Currently coinoutlet.com don't seem to support wallets themselves, they expect you to get a wallet elsewhere and then use it with the ATM. Their instructions say

Buying bitcoins

Before you buy bitcoins, you first must get a wallet. Wallet generating sites can help you do this, but the easiest option is to download a wallet app onto your phone from Coinbase, Blockchain.info or any other wallet from app store.

With a wallet ready, hit the start key on the ATM screen and expose the wallet’s QR code to the machine’s scanner. And your address will appear on the screen.

Insert the bills equivalent to the number of bitcoins you want to purchase. The exchange rate is displayed on the screen, as well as the amount you are about to purchase.

By clicking on the send button, you will finish the transaction. You will receive a notification both on the ATM screen and your phone confirming reception of bitcoins into your wallet. Don’t forget to collect your receive.

If those are the instructions you followed, it is more likely that the receipt just shows the address of your wallet. You need to find your wallet. Your bitcoin-address is of no help in finding your wallet and cannot be used to regain control over your bitcoin.

is it even still valid. It is from 2015.

The Bitcoins you purchased are probably still there unspent. So long as neither you nor anyone else moved the bitcoins elsewhere.