So I've seen a ridiculous amount of people who can't connect their old accounts. I may have a possible solution for some of you.
If you're anything like me, the last version I played was 2018. I saw the new one in app store and figured was an updated version of the same. (Yeah right). Didn't realize it was same people but a different app. I opened it and to my shock and agony it would not connect to Facebook from my 2018 version and instead wanted me to start a new account. Heck no!
So uninstall and reinstall all day long. It probably won't do anything but make you mad. So being the guru that I am I decided after some research what was best, at least for me. I saw lot of people telling you to contact customer service also. But if you are like me then you probably don't remember your username either. But there is hope.
What I personally did was googled the older version I had- MLB Tap Sports 18 apk. It may still be in play store also. And lo and behold what happened? It allowed me to login with my old account via Facebook connection. After finding that I sent a message to customer care within the 2019 version with the quick customer care chat access and sent screenshots of my old account, along with my email to verify my google account and a couple of screen shots from that google account for the old version 18 of the game I last used.
There isn't a lovely feature to bind the old to the new for most and that's where the hang up is. If this is your issue also then it's not a hard fix. Look for any older version you remember using. I sure wasn't going to start a new one after the time and fee bucks I had spent on it. The older versions didn't have email or website sign up like the newer ones have so that's where a lot of us are having trouble.
So for you and me it can be as simple as sending a message. Customer service can fix that. So don't give up! I hope this helps. Please keep me posted on yours in the comments.