How to purchase an IPad Pro with using e-gift voucher?

See this support article:--> What type of gift card do I have? - Apple Support

And/or this:--> Redeem your Apple Gift Card - Apple Support

Regards

Giulio

Your “e-gift voucher” may determine where (and how) the voucher can be redeemed. Apple will routinely accept Apple Gift Cards, but might not accept vouchers from third-party sources.