Infected Roots not working

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I am new to this, so I might have started this in the wrong section but please bear with me.

I have put points in Infect Roots, but most of the time the skill is shown as red, as in I cannot use it.
Does anyone know what I am doing wrong or is this a known bug?

It seems to work up until I use a auto attack or another ability and then it just stays red after that and
cannot use it anymore