How to fix such an error?

ERROR: Unable to find dependency[10.2.4] in paths ($SDK\extras\android\m2repository, $SDK\extras\google\m2repository, $SDK\extras\android\m2repository, $SDK\extras\google\m2repository).[10.2.4] was referenced by:
at System.Environment.get_StackTrace() in /Users/builduser/buildslave/mono/build/mcs/class/corlib/System/Environment.cs:line 227

I think u imported a *.dll (System.Environment) which needs a other *.dll. Try to reference the other *.dll by copying it to the same folder in ur assets.

Hopefully it works.

I had same question when I using the google leaderboard !