active directory User Manager

I don't know how your AD is structured but in ours it goes something like this

let
    Source = ActiveDirectory.Domains("domain.net"),
    domain.net = Source{[Domain="domain.net"]}[#"Object Categories"],
    organizationalPerson1 = domain.net{[Category="organizationalPerson"]}[Objects],
    #"Removed Other Columns" = Table.SelectColumns(organizationalPerson1,{"organizationalPerson", "distinguishedName"}),
    #"Expanded organizationalPerson" = Table.ExpandRecordColumn(#"Removed Other Columns", "organizationalPerson", {"mail", "manager"}, {"organizationalPerson.mail", "organizationalPerson.manager"}),
    #"Expanded organizationalPerson.manager" = Table.ExpandRecordColumn(#"Expanded organizationalPerson", "organizationalPerson.manager", {"mail"}, {"organizationalPerson.manager.mail"}),
    #"Filtered Rows" = Table.SelectRows(#"Expanded organizationalPerson.manager", each [organizationalPerson.manager.mail] <> null)
in
    #"Filtered Rows"