Destination tag isn't always required and that's why Bitsler performed your withdrawal without destination tag.

Destination tag is usually required when you send XRP to exchanges or custodial wallets (or any other third party)
Exchanges and custodial wallets usually give all users a same address and give a unique destination tag to each of them to know who has deposited.
Since Bitsler cannot know what's the policy of the wallet or exchange you are withdrawing to, they cannot know if a destination tag is required.

Note that the fund is now in the address you entered as withdrawal address. But the service you are using has't credited your balance because they don't know the fund has been deposited by you.