Robux gift cards (and all other Microsoft Rewards items) are region- and catalog-dependent and can go out of stock or be removed from a specific country’s catalog at any time.
Czechia is listed as a supported Microsoft Rewards region, so the program itself is available there, but not every reward is guaranteed to be continuously or permanently offered in every region. When a specific gift card disappears from the Redeem page or can no longer be set as a goal, this indicates it is currently unavailable in that region’s catalog rather than a bug.
To confirm what is currently available in Czechia, sign in to the Rewards dashboard and check the Redeem page directly; only items shown there are redeemable with points in that region.
References: