[root@slave02 /]# snap install core
error: system does not fully support snapd: cannot read the value of fs.may_detach_mounts kernel
parameter: open /proc/sys/fs/may_detach_mounts: no such file or directory
The above is my error message。My Linux version is CentOS Linux release 7.3.1611 (Core) ,Help me, Thanks!
That isn't the appropriate forum for questions related to snap installation, but I can still tell you that the CentOS version of 7.3.1611 is quiet old. You should upgrade it.
Yeah, for an OS that old it might work better to use the pip installation. Or, use a different client with minimal dependencies that you can just copy onto the server and run, like lego (just go to the release page and download lego_v4.17.3_linux_amd64.tar.gz), or maybe one of the shell-script based clients.
Though if your OS isn't getting security updates, then adding a certificate won't magically make it "secure".
I'm not sure; it's been a long time since I've used a CentOS/RHEL 7 system, and I haven't tried installing pip or certbot on it. Like I said, with something that old you're probably best off using lego or acme.sh (at least until you can move to an up-to-date server).