Free Trial of Apple TV from Barclaycard

It was not clear to me why you got that error message. Could it be about your account not being valid? Could it be that you tried multiple times? Did you try to redeem one of the other codes earlier? There should be four codes, only one of them for Apple TV+.

1. Do you (still) have an Apple ID account, one that is accessible? One that you can use for this offer? With current user details and a payment method (Barclaycard perhaps)?
2. Did you use that account to try to redeem this offer?
3. Barclays Bank makes this offer. They are the source for information about this offer and how to redeem, including FAQs. See the quoted link to their web page about these offers.
4. You will not be able to get new codes for any reason, per Barclaycard’s terms and conditions.
5. “doesn't answer my question” → Only question in original post: “Is this a scam?” → No, it is not a scam.
6. From the FAQ: “Queries on Apple services or the redemption process If you have a query when redeeming the Apple service(s) or on the length of the offer you’ve been given on the service(s), please contact Apple Support at https://support.apple.com. You can also check our terms and conditions for full details on the offer.”
7. This forum is a user-to-user board. None here are Apple Support staff. To contact the support staff: Get support - Apple Support (email/chat/callback) or Contact Apple for support and service - Apple Support (phone numbers).
8. “What does this actually mean for someone who hasn't used Apple for several years?” → Apple doesn’t use QR codes for their offers themselves, so the procedure is somewhat unusual and specific to certain partnering companies that make offers for Apple services as part of their promotions. Ask Barclays Bank or Apple Support for redeeming details that are not clear from the offer page.
9. The codes for Apple News+ and Apple Arcade may not be valid without an iOS device (e.g. iPhone). Apple Music should be valid on Android with the Apple Music app. Apple TV+ would be accessible on browsers or devices with the Apple TV app.

See: Get up to five months free of Apple subscriptions with Barclaycard - Barclaycard

Terms and Conditions

5. Getting the codes. If you’re a new customer, we [Barclaycard] will send you the unique codes automatically.

Otherwise, we may send you details on how you can get the codes or put the information on our website. We’ll use the contact details that we hold on your Barclaycard account for any communication that we send to you. We won’t be able to reissue codes so please keep them safe.

Codes are personal and can be use once.

You must have an Apple ID account to use the subscription on. If you don’t have one yet, then you can create one. How to create a new Apple ID - Apple Support. They’ll want some personal details, a verified email address (respond to test message), and a valid payment method for any subscriptions and possible future purchases that you might make through that account.