You can only leave reactions to messages sent with someone you're chatting with. This requires them to also be on a compatible Android phone and have their chat functions enabled.
Otherwise, you may receive reactions from iPhone users, but you cannot send reactions. This is enabled in the menu.
Do you happen to know what's going on there?
I don't have a firm reason for the inconsistencies. I would just recommend turning your chat settings off and then turning them all back on!
Will Samsung (or Android in general) ever have the reaction to text messages like iPhones do? Just this missing feature alone makes me want to get an iPhone.. I am VERY jealous that my iPhone friends can do this but no one in my family can because we all have Androids. NO, I do not want to download Google chat or any other additional app to do this. It seems SIMPLE.. If Apple can do it, why can't Android on REGULAR SMS texting?!?!?!