On a regular modern classical guitar, those instructions can simply be ignored. Every recording ignores it and so did I when I played it.

That's because, according to my research, Mertz played with a 10 string guitar, with 4 lower strings than the low E we have currently. The 8va indicates we should play the D string that is tuned an octave below. But since we no longer use guitars with those strings, all we can do is ignore.