Hi,
I have for a while tried to install new Teams with PSADT 4 but I cannot get through with it. Can anyone help out? Is this totally wrong Start-ADTProcess -FilePath "teamsbootstrapper.exe" -ArgumentList "-o -p MSTeams-x64.msix"?
Looking at the Microsoft guidance here it looks like you are on the right path:
Without seeing your PSADT log, I can only guess, but I suspect it may be the path to the MSTeams-x64.msix file that can not be resolved, maybe try using the PSADT files path here. e.g.
Start-ADTProcess -FilePath "teamsbootstrapper.exe" -ArgumentList "-o -p $($adtSession.dirfiles)\MSTeams-x64.msix"
To build on Adrian's suggestion, I think you'll want:
Start-ADTProcess -FilePath "teamsbootstrapper.exe" -ArgumentList "-o -p `"$($adtSession.dirfiles)\MSTeams-x64.msix`""
(Note the extra quotes around the path to the .msix file.)
Thanks Mitch 
Knowing that it's often me that replies to posts here about missing quotes or missing escaped quotes... I completely missed it this time
Tried it now but no difference, unfortunately. Also did a uninstall first but it didn't change anything.
Found this now, but I guess thats what your command does - $adtsession...?
Can you have a look at the PSADT log, it should give you some clues as to why it's not working.
I'm speculating, but I reckon it cannot find the .MSIX file (you haven't stated where you have this located), if you had it in the 'Files' subdirectory of the PSADT deployment (where it 'should' be), the above command should work, but if it's else where, the path in your command $($adtSession.dirfiles)\MSTeams-x64.msix will need correcting
Ah... just spotted it...
PSADT may not be finding the bootstrapper, try this
Start-ADTProcess -FilePath "$($adtSession.dirfiles)\teamsbootstrapper.exe" -ArgumentList "-o -p `"$($adtSession.dirfiles)\MSTeams-x64.msix`""
This assumes both the files (teamsbootstrapper.exe & MSTeams-x64.msix) are in the Files subdirectory
Ah sorry, should have mentioned that when I run this command (as a test) to remove Teams first - just to avoid any issues and have a clean setup Start-ADTProcess -FilePath "teamsbootstrapper.exe" -ArgumentList "-x" it successfully executes so teamsbootstrappe is found. I tried the last one now but again exit code 2.
Found the error whilst testing it manually. The options -o and -p needs to be the other way around so Adrians line Start-ADTProcess -FilePath "teamsbootstrapper.exe" -ArgumentList "-p -o "$($adtSession.dirfiles)\MSTeams-x64.msix"" is the one working, note the -p -o. 
Sorry @Robberg74, I need to step away from this discussion, You clearly had provided the right info in your posting, I'd just not read all of it 
Your screen grab of the log clearly shows (on the 2nd line) it finds the bootstrapper
I should have noticed this.
I'm unsure what's going on here ATM, I'll leave someone else with more experience to advise.
Good luck
Most things usually end up being a layer 8/PEBKAC issue. Glad you got there boss!
Excellent, glad you found it
TBH that's bad form from the Microsoft developers, it shouldn't really matter which way round arguments are passed.
Good to know!
Thanks guys.
I was working on this issue also and was about to submit a post.
I can confirm i did also get the code to work
Start-ADTProcess -FilePath "teamsbootstrapper.exe" -ArgumentList "-p -o `"$($adtSession.dirfiles)\MSTeams-x64.msix`""
just testing i also discovered this would also work (dunno the caveats of daisy chaining multiple "text" )
Start-ADTProcess -FilePath "teamsbootstrapper.exe" -ArgumentList "-p -o ""$($adtSession.dirfiles)\MSTeams-x64.msix"
