The ShopBack button may not be available for all merchants as it requires the merchant's approval. If the extension is not able to turn green on the merchant's site, it means that the extension is not supported for the merchant. It means that you have to click from the ShopBack website to the merchant's page for Cashback eligibility.
On another scenario, during payment the ShopBack Button will maintain in red due to tracking cookies would not be able to follow through for security purposes.
Also, if you have any adblockers installed, do note that the ShopBack Button will always maintain in red due to tracking cookies are blocked!