The polyfit function also needs to know what degree of polynomial you want to fit:
where ‘n’ is the degree (1=linear, 2=quadratic, etc.).
Also, in order to plot it, you will need to evaluate it first with the polyval function.
EDIT — (19 Oct 2020 at 198:10)
Since your data are linearly related, the full code would go something like this —
REGRE_FG_fit = polyval(REGRE_FG,FGEl(86:99));
plot(FGEl(86:99), REGRE_FG_fit, 'r')