您已经使其变得非常复杂.把事情简单化.试试这个(未经测试)

You have made it very complicated. Keep it simple. Try this (untested)

Private Sub LoginButton_Click()
    Dim Username As String
    Dim password As String
    Dim passWs As Worksheet
    Dim rng As Range
    Dim CorrectDetails As Boolean

    Username = UsernameTextbox.Text
    password = PasswordTextbox.Text

    If Len(Trim(Username)) = 0 Then
        UsernameTextbox.SetFocus
        MsgBox "Please enter the username", vbOKOnly, "Required Data"
        Exit Sub
    End If

    If Len(Trim(password)) = 0 Then
        PasswordTextbox.SetFocus
        MsgBox "Please enter the password", vbOKOnly, "Incomplete Entry"
        Exit Sub
    End If

    Set passWs = ThisWorkbook.Worksheets("Users")

    With passWs
        lrow = .Range("A" & .Rows.Count).End(xlUp).Row

        For i = 1 To lrow
            If UCase(Trim(.Range("B" & i).Value)) = UCase(Trim(Username)) Then '<~~ Username Check
                If .Range("C" & i).Value = password Then '<~~ Password Check
                    CorrectDetails = True

                    '~~> Admin is True
                    If .Range("D" & i).Value = "True" Then
                        '
                        '~~> Do what you want
                        '
                    Else
                        '
                        '~~> Do what you want
                        '
                    End If

                    Exit For
                End If
            End If
        Next i

        '~~> Incorrect Username/Password
        If CorrectDetails = False Then
            MsgBox "Invalid Username/Password"
        End If
    End With
End Sub

我的假设

在用户"表中，Col B具有用户名，Col C具有密码，Col D具有管理员值.如果没有，请根据需要修改以上代码.

In sheet "Users", Col B has username, Col C has password and Col D has Admin values.. If not then please amend the above code as required.