There are at least 3 ways to make new tiddlers in TW. Of these, only the ActionCreateWidget will automatically prevent overwriting titles. ACW
also has the advantage that it doesn't open your tiddler in edit mode.
In your original you had text="{{transclusion}}" . You can not do that. You must use text={{transclusion}} . Otherwise, the actual text will be "{{transclusion}}" and not the transcluded content.
In your example, <$action-sendmessage was not closed on the end with "/>". Always do a double-check to make sure that all tags are matched by their anti-tag
(e.g. <mytag> .... </mytag> or that a single tag is closed on the end with "/>" .
In your original example, I did not know what $:/tags/Template did or what it's contents were. So I removed it for my test.
This version works, though I don't know if it does exactly what you intended:
\define testknopf(Kennzeichen Sache)
<$button>Mein Knopf
<$action-createtiddler $basetitle="$Sache$ - Jahr" tags="$Kennzeichen$ $Sache$" text={{Benzin - Jahr}} />
</$button>
\end
<<testknopf ImportantTag Thing>>
If you press the button twice, it will make a new tiddler based on the name, but will not over write the first one.
HTH