I built an android sample project (HelloARUnreal) and had this problem several times. I tried to rebuild Unreal Engine (https://github.com/google-ar-unreal/UnrealEngine), but that doesn’t help.
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HelloARUnreal-armv7-es2.so
F:\UnrealEngine\Engine\Plugins\Runtime\GoogleARCore\Source\GoogleARCoreBase\Private/GoogleARCoreAPI.cpp:307: error: undefined reference to 'ArConfig_setFocusMode'
Do not repeat my mistakes, follow the instructions carefully:
Prevent setup from overwriting the included Google ARCore binary files with the older files provided by Epic. Select n when you see Would you like to overwrite your changes (y/n)?.