This directory (and version of CentOS) is deprecated. For normal users,
you should use /7/ and not /7.6.1810/ in your path. Please see this FAQ
concerning the CentOS release scheme:

https://wiki.centos.org/FAQ/General

If you know what you are doing, and absolutely want to remain at the 7.6.1810
level, go to http://vault.centos.org/ for packages.

Please keep in mind that 7.6.1810 no longer gets any updates, nor
any security fix's.