You've misunderstood the notation. `2E-51` is not a negative number, but machine notation for the number $2 \times 10^{-51}$, which is equal to $$0.000000000000000000000000000000000000000000000000002.$$ This corresponds to the probability of observing a value of the $F$-statistic at least as large as $69.1$ by random chance, assuming the null hypothesis is true (in your case, no effect).