Q: Why Does Apple Ask for Credit Card Details To Install FREE Upgrades, Then Refuse To Accept The Payment Details Entered?

You don't have to be signed into an apple id to download 10.11. But if you are trying to create a new apple id make sure your signed into the right region. For instance this is the us site. If you try to use your info from the uk that might be the cause of your issue.

Sou_Z wrote:

 

I'm trying to download the supposedly "Free Upgrade" to install OS X El Capitan in my MacBook Air and Apple has been making me jump through hoops for the past 3 hours!

I'm just guessing but had you contacted Apple support you wouldn't have spent 3+ hours avoiding contacting Apple support.

You don't have to be signed into an apple id to download 10.11.

Not sure where you got this information. Since all purchases/downloads are tied to the Apple ID used to obtain them, yes, you do need to be signed in.

To test, I signed out and opened the app store. I then clicked on "get" for El Capitan and got this log in window:

Is your Pay Pal account linked to a credit card?  If not it's not acceptable.  the following is from this Apple document: Payment methods you can use in the iTunes Store, App Store, and iBooks Store - Apple Support

The available payment methods vary by country. You can typically use these methods:

• Credit cards
• Debit cards
• Content codes
• Store credit, including iTunes Store Gift Cards, Gift Certificates, and Allowances
• PayPal (requires the account to be linked to a credit card)
• China UnionPay (requires SMS PIN for setup)
• Mobile phone billing (through supported carriers)
• Bank Cards (China only)¹

Your right i was thinking about updating via the app store which you can do without signing in. Now instead of just correcting me how about helping the OP.

This might help, follow the link :

Create an iTunes Store, App Store, or iBooks Store account without a credit card or other payment method - Apple Suppor…

i was thinking about updating via the app store which you can do without signing in.

No you can't.  To update an app from the App Store you must be signed in to the Apple ID account that was used originally  to purchase that app.  Once you sign in you remain signed in until you sign out. 

Not talking about an app. The OP is talking about 10.11 not any apps. If you clean install 10.11 from usb and don't sign in with your apple id you can still update 10.11 without being signed in. Talking about just updating the OS not any apps.

Yes, I know.  But the OP is  not referring to a USB, but downloading from the App Store. The USB reference has no significance in this topic.

One additional idea: do you have a payment method set up in your iTunes account? That is the "central" bookkeeping place for the Mac app store, the iOS app store, and the iTunes store.