One of these users said
I’m so glad I’m not the only one thinking this. Samsung doesn’t listen to its base. We want less bloat–now we have more and a dedicated button that does nothing in th U.S, expanding bloat to hardware. Samsung ruined the perfect phone with Bixby. I preordered and returned promptly.
Their is no solution to fix Bixby button not working problem except downgrading to previous version of android or waiting until Samsung solve this problem by fix this bug.
some users suggest to Double click the Bixby Button or press the Bixby button real quick just once and then wait a second. and worked for some of users and the other said that method not working with them.
shasypalmtree an user at Samsung community suggests:
We hope and wait an update to solve this problem but as you know their are a lot of problems come with Samsung Galaxy S8 ans S8 plus.