A family of Microsoft word processing software products for creating web, email, and print documents.
Although the macro recorder is useful for finding out what commands are (maybe) useful, there are lots of problems with what it produces. (For examples, see http://www.word.mvps.org/FAQs/MacrosVBA/ModifyRecordedMacro.htm.)
I don't have an explanation for the error message, but there are several other things that need attention in the recorded code, and there aren't any errors when those things are fixed.
First, there seems to be some confusion about documents vs. templates. The Documents.Open method is meant to open a document (*.doc, *.docx, or *.docm). If you use it to open a template (*.dotx or *.dotm), it will open the template itself for editing, which usually isn't what you want to do. If what you really want is a new document based on a template, use the Documents.Add method instead.
Second, it isn't necessary to use the ChangeFileOpenDirectory command; just include the path as part of the file name.
Third, when you do use one or the other of these methods, you can omit all of the arguments that have their default values.
Finally, if you want the userform to close when the button is clicked, the last command in the Click procedure should be Me.Hide.
So, if you want to open an existing document (*.docx), use
Private Sub CommandButton1_Click()
Documents.Open FileName:="C:\File Location\General Letter.docx", AddToRecentFiles:=False
Me.Hide
End Sub
If you want to create a new document based on a template (assuming the template is in the User Templates location), use
Private Sub CommandButton1_Click()
Documents.Add Template:=Options.DefaultFilePath(wdUserTemplatesPath) & "\General Letter.dotx"
Me.Hide
End Sub
[Notice the backslash at the start of the template's name, needed because the result of the DefaultFilePath doesn't end with a backslash.]