I'm trying to merge a single scene by launching the Unity YAML merge tool from SourceTree and I get this error :
Couldn't locate merge tool to handle extension tmp in [Path to
Hope it's not to late for an answer but I've encountered the same issue some hours ago and figured out how to fix it.
You simply open the "mergespecfile.txt" and fill in a fallback mergetool by adding a path to a merge tool of your choice. Right under the following part:
# # Default fallbacks for unknown files. First tool found is used. #
add a path to your merge tool like:
* use "C:\path\to\tool\p4merge.exe" "%l" "%r" "%b" "%d"
In general the error occurs because unitys merge tool tries to find an alternative mergetool for files it can't handle itself and if all default tools weren't found it throws the error above.