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That looks like it should be part of a catch statement:

try {
....
}catch (UnavailableArcoreNotInstalledException) || (UnavailableUserDeclinedInstallationException) {
...
}...

That message says that the compiler can not find where the symbol mentioned in the message has been defined in scope where it is being used.
Roughly, in scope means within the same pair of {}s

It...