If it's an Android, usually with Nintendo's mobile games "not compatible" is code for "It failed the SafetyNet check".
Basically, that means they detected that it's rooted, or in some extreme cases even detected that the bootloader is unlocked even if your phone is not rooted (Dragalia Lost started checking for that).
They started doing this in a surprise update on Pokemon GO years ago, and then every other Nintendo mobile game has been using it since. Nintendo thinks they are being clever by making all of their Android versions of games use SafetyNet to check if a phone has been rooted, and then claim benign errors like "Authentication Error" or "Device Incompatible" thinking they are being sneaky by not just flat out admitting they refuse to let the game run because your phone is rooted.
They normally don't give a damn about actually listing specific devices as being compatible or not outside of a minimum OS version, but if your phone is too hold to pass that then it would not run the game above 2FPS anyway, so any message about your device not being compatible is almost always because it's rooted.
If your phone is Android and rooted, try using Magisk to hide root from them, use a SafetyNet checking app to see if SafetyNet is passing.
