I tried to build KDE Frameworks 5. I am receiving this message : The session is not registered with logind "No such file or directory". How do I register the session ?

Unfortunately KDE Frameworks 5 is still not officially added. The current information about this and other KDE5/Plasma software packages is geared solely towards systemd. Work is going to need to be done to remove or supplicant alternatives to systemd.

If you need logind support, or at least possibly a shim to test, try loginkit and ConsoleKit2. I have a copy of loginkit's recent git pull for version 0.1 in my bitbucket repository and the Slackbuild used can be translated back to BLFS. The info file for ConsoleKit2 has the link for 0.9.2's release. Loginkit works with ConsoleKit2 to emulate functions from logind. Be advised, loginkit can use Linux-PAM unless you use the patch I have up to use it without PAM.

Unfortunately, you might want to look into a sysvinit bootscript for ConsoleKit to be started at boot rather than at launching your desktop window manager for best results. LFS does not have one.

@ReaperX7: Is not there systemd version of LFS? So why you guys say programs won't run because systemd is not available.

The systemd book is set up similar to, but equally different from the sysvinit book. There are software packages that can be built between both, but there are some that can't without either replacement software or patches.

Equally, the OP did not address whether or not he was using the sysvinit or systemd book, and normally, any questions regarding the systemd book are usually labeled strictly for systemd help. We can only assert that the sysvinit book is being used because it is the standard book in the LFS systemology and tree unless said so.

The original post can only deduce that he was trying to build a package, somehow built it successfully, but then received an error when the program was ran for whatever reason.