I'm trying to make 50 Key giveaway but getting Error

it's not level based, it's gift based
"When you create giveaways and your gifts are received, this number will increase."

That based on number of gift you already give, you get one extra slot every 3 giveaways complete, so for 50 slots, you need to have done 141 giveways complete.

https://www.steamgifts.com/about/faq
Developers
I'm a game developer or publisher looking to promote my game through giveaways. How do I get started?
If you're looking to promote a game, the best option is to create a featured giveaway. These giveaways are displayed at the top of our homepage, and receive the highest visibility. Featured giveaways are created in the same manner as every other giveaway, and our site will automatically feature giveaways on our homepage if they contain 50 copies of a game or greater, and if they're ending within four days. We feature up to two giveaways at once, and if more are available, then two will be displayed at random, with preference given to giveaways of higher copies. Please note, the giveaway will only be displayed to users that have permission to join, and if you choose to limit your giveaway to a particular Steam group, it will only be visible on the homepage to members of that group. If the limitations on your account do not allow you to create a giveaway for 50 copies or greater, then please contact support. We'll upgrade your account to developer status, allowing you to create giveaways up to 100,000 copies.