I believe as of version 5.2.3 of the pf2e system, the flat check portion of the module doesn't work anymore. Idle's PF2e Perception module faced a similar issue which he has fixed since, in case you want to take a look. Either way, in order to reproduce it, you just try pressing any of the flat check buttons above the chat box and the error will occur. Here is the console error
Uncaught (in promise) TypeError: game.pf2e.Check.createResultFlavor is not a function at rollFlatCheck (module.js:272:42)
I believe as of version 5.2.3 of the pf2e system, the flat check portion of the module doesn't work anymore. Idle's PF2e Perception module faced a similar issue which he has fixed since, in case you want to take a look. Either way, in order to reproduce it, you just try pressing any of the flat check buttons above the chat box and the error will occur. Here is the console error
Uncaught (in promise) TypeError: game.pf2e.Check.createResultFlavor is not a function at rollFlatCheck (module.js:272:42)