Hi everyone, I would like to fit my data with the function describe on the file

See if the attached helps. Have concentrated on your question about y being a scalar as you expected a vector. The magic is done by using the vectorize command from the matrix palette.

Maybe looking at http://communities.ptc.com/message/197133#197133 can help to get a better fit. On the other hand, as far as I see, you are trying a simple exponential fit (a*e^b*x) and I doubt that thats appropriate.

I was confused by some action you did in that sheet and its a little bit messy, so maybe I overlooked some detail.