Can't update iPad OS

I have two identical older iPads, ME276LL/A. I am able to run OS 12.4 on one. The other is at v. 11.3 and refused to allow an update. I go to Software Update, it says Unable to Check for Update? I've tried repeatedly and restarted the device a few times. I've never heard of this before! Can't update to the latest that this device will allow. The reason I need to is because I need to run a business app that won't even show up in the App Store search in the current system. The other iPad running 12.4 does show the app and allowed me to download it. What's this all about? Thanks.