That's pretty slick that Life360 exposes that value.
As for your rule, it's tough to tell with your redaction there, but it appears you have the rule backwards, You want "If battery level < 45 is true then turn on plug". From what I can read you are turning on the plug if it'snot less than 45 (you'd also need to expand the rule to turn off at your desired level of 85%)
I'm not exactly what the trouble is or what the "can't" stuff is about, but having "false" instead of "true" is giving you the opposite behavior of what you are after. The rule screenshot says "if the power is less than 45 percent is not true then turn on" and what you want is "if the power level is less than 45 percent then turn off else if power is greater than 85 then turn off"