Populate the selected cells value for command button email

Please try this solution.

Private Sub CommandButton1_Click()
'Updated by Extendoffice 2017/9/14
    Dim xOutApp As Object
    Dim xOutMail As Object
    Dim xMailBody As String

'    On Error Resume Next       ' hiding the crash won't prevent it
    Set xOutApp = CreateObject("Outlook.Application")
    Set xOutMail = xOutApp.CreateItem(0)
    xMailBody = "Hello Sir/Ma'am," & vbNewLine & vbNewLine & _
              "You are receiving this email because your Government Travel Card Statement of Understanding and Training Certificate has or is about to expire.  The Training is required to be accomplished every three years and the Statement of Understanding resigned every three years and upon arrival at each new duty station." & vbNewLine & vbNewLine & _
              "Attached is the SoU that needs to be completed and signed by both the member and the member's supervisor. Please follow the below steps to access the Training:" & vbNewLine & vbNewLine & _
              "-Go to the TRAX website:   https://www.defensetravel.dod.mil/Passport/bin/Passport.html" & vbNewLine & vbNewLine & _
              "-CAC Login" & vbNewLine & vbNewLine & _
              "-Click the Training Icon at the top of the page" & vbNewLine & vbNewLine & _
              "-Select the View All radio button" & vbNewLine & vbNewLine & _
              "-Select Launch for the Programs & Policies - Travel Card Program (Travel Card 101) [Mandatory] Course" & vbNewLine & vbNewLine & _
              "-When completed, format certificate to Landscape and save as PDF" & vbNewLine & vbNewLine & _
              "Once both documents are complete, respond to this email with both documents attached so we can update our records.  This is mandatory and failure to accomplish can result in temporary closure of your GTC account." & vbNewLine & vbNewLine & _
              "If you have any questions or concerns please let me know." & vbNewLine & vbNewLine & _
              "Thank you"
'    On Error Resume Next       ' hiding the crash won't prevent it

    With xOutMail
        .To = Recipients(Selection)
        .CC = ""
        .BCC = ""
        .Subject = "GTC SoU & Training Certificate Expired"
        .Body = xMailBody
        .Display   'or use .Send
    End With

    On Error GoTo 0
    Set xOutMail = Nothing
    Set xOutApp = Nothing
End Sub

Private Function Recipients(SelRng As Range) As String
    ' 19 Sep 2019

    Dim Fun As String
    Dim Cell As Range
    Dim Tmp As String

    On Error Resume Next        ' return a blank string if nothing was selected
    For Each Cell In SelRng.Columns(1).SpecialCells(xlCellTypeVisible)
        Tmp = Cell.Value
        ' test if the cell appears to contain an email address
        If InStr(Tmp, "@") Then
            If Len(Fun) Then Fun = Fun & ";"
            Fun = Fun & Tmp
        End If
    Next Cell

    Recipients = Fun
End Function

Selection.Value can be a string or an array, depending upon what you selected. If you selected more than one email address the array must be converted to a string which separates the recipients' addresses with semicolons. The function Recipients I added to your code does this. The string it returns is then assigned to the mail's To field.

I took exception to the liberal use of On Error Resume Next in your code. Essentially, if the email can't be created for whatever reason your code will silently do nothing. In such a case one tends to look at the screen in unbelieving stupidity and press the button again and again. Removing the On Error Resume Next will cause the program to crash with some kind of error message. Whether or not that is more helpful is debatable but I think it's better than a blank screen. Reinstate the two instances of the offending code if you so prefer. The better way would be to trap the errors and issue an informative message. But that is the subject of another question.

The same applies to your wish to attach a document to your mail. The answer has no room here.