Why it is not available for the 10.13 Mac owners to download big sur through the links on Apples support page ?

MacBook Pro 13″, macOS 10.13

Check if your macBook Pro is eligible for the update -> macOS Big Sur is compatible with these computers

macOS Big Sur is compatible with these computers - Apple Support

I think if your equipment is not eligible for Big Sur you won't even see a "download" option. A numeric error message like that strikes me as being a more technical issue and would be best handled by Apple. Refer to this document for ways to contact Apple ➞ Choose your country or region - Official Apple Support

Select your country (also look for "other" regions), then a product. If you don't see one that handles your issue then keep experimenting with selections until you reach one that gets you a chat session or a telephone call and get the representative to redirect you.

or:

Contact Apple for support and service by telephone ➞ Contact Apple for support and service - Apple Support