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If "a" has units of length then the two arguments you pass to the min function have different units.

As Richard already stated you are comparing quantities of different dimensions. The first argument of max() has the units Newton * secondss but the second hast unit Newton. This will not work.

Are you sure about your function Phi(a)? At the end you are taking the max of one single value which works, but does not make much sense. Which dimension should Phi(a) return? The way you wrote it it returne "time" because at the end you added minutes. Was this done on purpose? The expression for PhiP.n(a) would work if Phi(a) is dimensionless.

WE