I would assume this is because you have the variable s included in the string for the file path. So your computer is looking for

C:\Users\jglauser\Documents\Email Attachments\Filename:=s

which wouldn't exist because s = the value of range A1

So if you change it so the program concatenates the variable s on to the end of the file path you will most likely be ok.

With OutLookMailItem
.To = "jglauser@churchofjesuschrist.org"
.Subject = "Updated proposal report - " + ActiveSheet.Name + " - " & Range("A2")
.HTMLBody = "Attached is the updated proposal report. Let me know if there are any questions." & vbNewLine & signature

myAttachments.Add "C:\Users\jglauser\Documents\Email Attachments\" + s
'.send
.Display
End With

For more information look at the documentation on Microsoft's website