What does “agency” mean in this context?
There is a lack of agency here - a feeling that you have little control over your life and a willingness to blame everyone but yourself. - pg. 7 of Hillbilly Elegy by J. D. Vance

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'lack of agency' in this context means the individual lacks the ability to act independently and with free will. He lacks agency - he lacks the ability to do what he wants. He is constrained/limited by forces or people external to him.
https://en.wikipedia.org/wiki/Agency_(sociology)