It looks like you want to find the root of 1 + a1*p + a2*p^2 - z == 0
when p
has different values from the slider (after multiplying it by 100 ?) The first error you had due to how you over used /. The second issue is FindRoot
itself, says
The line search decreased the step size to within tolerance specified by AccuracyGoal and PrecisionGoal but was unable to find a \
sufficient decrease in the merit function. You may need more than MachinePrecision digits of working precision to meet these \
tolerances. >>
For the above, you have to change the guess value or look into the suggestions given above.
Manipulate[
2 + func[p*100, guess],
{{p, 10, "P"}, 10, 20, 1, Appearance -> "Labeled", ImageSize -> Small},
{{guess, .5, "guess"}, -1, 1, 0.01, Appearance -> "Labeled", ImageSize -> Small},
Initialization :>
(
func[p_, guess_] := Module[{a1 = .3265, a2 = -1.07, z},
z /. FindRoot[1 + a1*p + a2*p^2 - z == 0, {z, guess}]]
)
]