I cannot click on “Withdraw” while I have more than €/$/£ 10 or R100 on my Stuvia account. How is this possible?

You can only withdraw the money you earned with the sale of documents. The money you credited onto your Stuvia account, rewards or money you’ve been refunded in the case of a serious complaint, doesn't count as your earnings, and therefore there won't be possible to get via the ‘payout’ mechanism.