If you are trying to define the function SetFigureSavePath, the first input specified in the definition must be the name of a variable into which the first input argument with which the user calls it will be stored in the function's workspace. You're specifying an expression.
If you are trying to call the function SetFigureSavePath, remove the function keyword at the start of the call. In this case, passing varargin on its own may not do what you expect. You may want to turn it into a comma-separated list.
P = 'C:\Users\Brigid\Documents\Github\MERFISH_analysis\newPath';
[savePath, parameters] = SetFigureSavePath(P, varargin{:})
