This link is active
https://web.obarun.org/software/66/v0.5.1.1/66-enable.html
Thanks for pointing this out. Since 66 is undergoing development and there is one stable release 0.5.1.1 and several 0.6 releases, currently the one in obcore-testing repository is 0.6.1 and the documentation will soon change a little, please be patient.
https://web.obarun.org/software/66/ the main link for the documentation shows the current stable and changes of new versions.
Also % man 66-enable should give you the same manual as the version you have installed.
The introduction to 66 is also helpful for common tasks.
Please provide the output of % sudo 66-intree -g so I can tell what trees you already have set-up and please tell us what installation you did (base, openbox, xfce, plasma, jwm) or custom.
% sudo 66-enable -t root dhcpcd
It appears as you have no active/current tree for this to happen.
% sudo 66-tree -ncE root
will create at least one active/current/Enabled tree
When you don't specify a tree then the service is activate/enabled in the "current" tree.
Names of trees are arbitrary.