It isn't impossible to earn Robux, but since Roblox is a free-to-play game, it’s designed to encourage players spend their own money on microtransactions, rather than giving away items for free.

Steps for altering your avatar’s appearance in every Roblox experience, from adding and removing items to changing your skin ...

Although there are a lot of sites claiming to give away free Robux, there is actually only one legitimate way to get free Roblox money.