Could you please let me know your RSN?
Or even better, please submit a bug report if you haven't already (I searched through our JIRA database but couldn't find a ticket that's been created as yet), so that we can get a snapshot of your account in order to repro your state, and therefore construct a fix.
To submit a bug report, press the - button in the top right corner of your player chat window, and choose Report, then Report a Bug->Gameplay.
Had a look at the code for Yak Track. From the messages you're getting, it looks like it hasn't initialised on login.
Best guess I have at the moment for a user fix, without being able to replicate and debug your exact account state:
In Settings, Gameplay tab->Information Windows: Tick Guidance system hints
Press F1 to open Hero parent, go to Achievements tab.
Press the bottom centre button on the Achievement Paths window - see if that'll let you Skip Tutorial or otherwise cycle to the Burthorpe path book.
Ensure the green "Hide achievement paths you've completed" green tick box is ticked and the "Hide achievement paths with requirements you have not met" padlock tick box is unticked.
See what paths still need to be completed in the Burthorpe path book by opening the blue dropdown menu under the Achievement Paths heading.
Ensure you've completed the To Lumbridge!->I Need a Hero achievement, and any prerequisites for it. That's the last achievement you need to complete to be eligible for Yak Track.
For added insurance, ensure you're on a path book other than Burthorpe (eg Lumbridge or the generic Paths book), and aren't tracking any paths in the activity tracker (hover over the Hero button on your ribbon, choose Activity Tracker, right click and Remove any path achievements listed on the Activity Tracker panel) when you try to start Yak Track.
If I can get your account name from you, I'll be able to investigate further. A bug report is the best way to ensure the issue is tracked and can be resolved by the relevant team. :)
Your screenshots were very helpful, though. Thanks for those! ^.^