Set objSelection = objWord.Selection
should be
Set objSelection = objWord.Selection.Range
objDoc.Tables.Add Range:=Selection.Range
should be
objDoc.Tables.Add Range:=objSelection
If you are running the code from an application other than Word, Are you running this code from an application other than Word, change 'wdWord9TableBehavior' to '1'.

Thus change the lines below as shown

Set objSelection = objWord.Selection.Range 
objDoc.Tables.Add Range:=objSelection, NumRows:=3, NumColumns:=4, DefaultTableBehavior:=1
However the quoted error usually relates to the use of the userform itself and when it is unloaded in relation to the code. Unload the form in the main code when you have finished with it and not in the userform code.