we're all differently built, we have different skeletal and muscular strengths and weaknesses. to train parkour is to explore your own capabilities and to push your boundaries. each obstacle is an opportunity to address a new challenge, to figure out whether it is difficult mentally or demanding physically and to focus on what in you is lacking and train that. my point was more that you have no choice but to accept the things you can not change, and that makes them all but irrelevant. I am as tall as I am, an underbar is as challenging to me as it will be, focusing on my height or using it as an excuse doesnt get me between those rails.
I am afraid - try something easier: bigger space to underbar, less jump required, easier angle of entry
I am not strong enough - strength train: work on your kips, lache', work on front lever progression
I am too tall - not the problem, see above