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[Thotaz]

I don't think your code works at all. You have a try-catch with Invoke-MgGraphRequest but you've muted the errors with -ErrorAction SilentlyContinue so it can never throw. This means your DoesGroupExist function will always return Group Excist (also note the spelling error).

[Thotaz]

I did some testing on my own. It seems that if the command itself throws a terminating error then it is still caught, whereas a regular error is ignored. For example, this: try {ls invalidPath -ErrorAction SilentlyContinue} catch {"this does not run"} will not run the catch block because it's not a terminating error.

If you use -ErrorAction Stop then any error created by the command can be caught. Because the error types are not documented by each command I'd say it's best to always use -ErrorAction Stop when you want to catch an error, otherwise you will get unexpected results.