“不忍用杖” can be translated as "reluctant to use the cane" or "hesitant to apply the rod" (where "rod" can also metaphorically refer to punishment or correction).
In a more general or contextualized sense, depending on the nuance you want to convey, you might also use expressions like:
"Unable to bring oneself to use the cane"
"Feeling不忍 (reluctant/unwilling) to resort to corporal punishment" (if referring to a more literal or traditional form of discipline)
The most straightforward and commonly understandable translation in a general context would likely be "reluctant to use the cane."