I don't know off-hand. Something seems fundamentally wrong with your Apache or openssl setup. I think "SSLProtocol all" allows Apache to start but does not enable any protocols which is why SSL Labs fails. It means "all possible" but I think none work.
This is not really a Let's Encrypt problem but a general Apache install issue. I am not the best person to help with that. Perhaps another volunteer will help or try stackoverflow.
But, I would start by checking your openssl setup and version. It is what Apache mod_ssl relies on to provide those protocols.
But I'm wondering if there is a mismatch between the protocols that Apache version 2.4.18 supports (e.g., not SSLv2) and the protocols that OpenSSL version 1.0.2g provides.
Sorry for my delay in getting back. I decided to take yesterday off, and go out of my apartment, and, in fact, go out of town for the day. I got back late last night, and made sure that I was fully rested before coming back to this big SSL enigma facing me, so that I could apply my full attention to the intricate task of correcting my typo. (Full disclosure: earlier this week I had to change "TSL" to "TLS" in ssl.conf ...)
Many thanks, especially to rg305 and MikeMcQ, but, really, to everyone who has participated in this crowd-sourced-debugging.