Firstly, Set(var, value) is for global variables, which means you keep the variable active until you close the application. You should use UpdateContext({var: value}) if you want a context variable.
Secondly, you should use LookUp function instead of Filter here:
Set(varManager,LookUp(TR_PersonalDetails,TRID = SelectedReportTR.TripTypeID));
Then, you will only use varManager.Email and not Last(varManager).Email
The code will be clearer.
Finally, you should use ManagerV2 since Manager function is deprecated. For example:
Office365Users.ManagerV2(varManager.Email).displayName
Now, if you still have an error, maybe try to just put a label field in your app to show what is varManager.Email exactly to validate if your email has a good value. I think you should make some error handling by validate the email before passing it as a parameter to the connector.
I hope it helps!
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