Is there a phrase/idiom that refers to the irony of putting off a task or goal when it's easiest to achieve?
If you want a turn of phrase, you could negate a phrase that means doing the task when it is the easiest or most likely to succeed.
He didn't make hay while the sun was shining. or He didn't strike while the iron was hot.
Original phrases:
Make the most of one's opportunities while you have the chance. [Make hay while the sun shines]
To strike while the iron is hot is to act decisively and take an opportunity when it arises.
Both definitions from https://www.phrases.org.uk/