How to calculate Fst from AMOVA
I have never performed an AMOVA myself and I don't know what a genind object is. But ...

Error

Typically, "error" can also be called "residuals". In other words, it is the sum of squared deviation between the data and the regression line (just like in any regression model). You might want to learn a little bit about regression and ANOVA to understand your AMOVA.

Fst definition

From Nei 1973 (very good paper, I recommend reading it),

$$F_{ST} = \frac{H_S-H_T}{H_T}$$

, where $H_S$ is the average subpopulation variance in allele frequency and $H_T$ is the total variance in allele frequency.

What you probably got from the AMOVA output

You should have these info in your AMOVA output. If it only give you the total variance and the between group variance (which is what is used for the F-test), then just remember that the among variance is the total variance minus the between group variance.