Hi, I'm Bernard, a fellow Windows user. I'm happy to help!
Some rewards, like Spotify Premium, are either out of stock or unavailable in your region at the moment. There is no official announcement on when they will be restocked or become available again. Moreover, for further assistance, it's best to contact Microsoft Rewards support, as they specialize in this matter. You can reach them through the link below:
Microsoft Rewards Support: https://rewards.bing.com/support