Why is part of my balance not withdrawable?

Your balance on Dexsport is split in two. The withdrawable part you can withdraw at any moment. The non-withdrawable part has to be wagered first. The Wallet section shows both numbers for every token, Balance and Withdrawable, so you always know which is which. In the withdrawal window the first one is labeled Game balance, which is the same number under a different name.
 

Read more: Withdrawable and non-withdrawable balance