Question: What is my mistake in using fsolve?

Hi,
I want to solve equations (1) and (2) to obtain the values of certain quantities, for example x and f. In fact, both equations must be satisfied simultaneously for \tau and f. Since there are other parameters in these equations (for example \tau, M, etc.), and I do not know the optimal values of these parameters, I would like to solve equations (1) and (2) over given ranges for tau, M, and so on. I know that I should use loop commands, but I keep encountering errors. Please guide me.
fsolve.mw

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