That is not your low side port. The green cap is indicating the EVAP system and it looks like there is a tag on there as well that states this. That is for your emissions system.
So here are some guides that will help locate the low side port:
Below is a chart that will help with pressure readings once we get this and the other is a location on the lines where these ports should be.
Take a look at this and let us know if you have questions and then what your pressures are with the engine off and then with the engine on and A/C on.
Images (Click to enlarge)
Saturday, June 12th, 2021 AT 6:20 PM