START needs a first parameter of a title name. Try:
START "" "Path\Excelfile.xlsm"
Edit: It doesn't need a first parameter of a title name, but if you give it a first parameter in quotes, it reads that as if you are trying to set a title name instead of a program to run, so you have to work around that.
See the help with start /?
- the first optional parameter is a quoted window title.
C:\>start /?
Starts a separate window to run a specified program or command.
START ["title"] [/D path] [/I] [/MIN] [/MAX] [/SEPARATE | /SHARED]
[/LOW | /NORMAL | /HIGH | /REALTIME | /ABOVENORMAL | /BELOWNORMAL]
[/NODE <NUMA node>] [/AFFINITY <hex affinity mask>] [/WAIT] [/B]
[command/program] [parameters]
"title" Title to display in window title bar.