Hello and welcome to the wonderful world of ReScript!
I am assuming ReScript 12 and the most current stable rescript-vscode plugin version ( 1.70.0 as of today)?
Did you try formatting in a vanilla project?
npm create rescript-app@latest
Because I just tried and it worked fine to format the .res files in there. Usually formatting only fails for me if there is an parse error in the same file.
Please tell us more about your setup which is probably more complex than the basic example I suggested to try formatting on?