wanted to buy the new humble montly, have money on paypal, but

you could send someone the money via paypal and they can send you the monthly as a gift

You need to add an visa card to your paypal to buy an humble monthly.

I've linked a prepaid card to my verified paypal and it works for me.

Humble Monthly is a subscription. PayPal's subscription billing system require an instant backup funding source, so there's nothing that Humble can do about it.

It works too if you link a bank account to Paypal

Send me the money via Paypal and I'll gift you the bundle.

Purchase a gift subscription.

As stated before It is a subscription and paypal needs a backing source like a credit card or bank account linked to you paypal account. You can do a workaround though. Buy a 1-Month gift subscription., you will get the gift link in your email, then redeem the gift for your humble account.

I see you are in Portugal not UK was going to suggest Natwest Basic Account.
As they give you a Visa Debit Account and ya can ONLY spend what you Have & Don't Get in shit street
Would look for something similar in your country.
All the best with your future Artwork. : )

Did you try adding it as a gift? You CAN gift it to yourself.

Have a friend gift you the monthly and send the payment through paypal. Works for me