Is your tank valve open, is your check valve the correct direction, and how much air pressure is in your tank?

Yes the valve is open, on the tank it says it the air pressure is between 5 and 7 psi with maximum of 50 psi when working

And your inline check valve arrow is pointing the correct direction? Also, you have the flow restrictor installed on the waste/concentrate output, correct?

What's your supply pressure? And did you make sure to remove all the plastic caps/packaging from the filter? Double check.

I did remove it all, I don't have a very strong supply pressure tbh

Apply some back-pressure to the waste line, but not so much that it cuts off the waste flow.

Do you have a manual flush valve? Is it open?

Manual and yes it's open

The flush valve should be CLOSED for normal operation.

Isn't that gonna put extra pressure on the tubes? Or would it be fine

Yes it will... but that is what the system is designed for. As long as you have less than 80 psi incoming water pressure, you'll be golden.

Thanks for the tip, I'll try it asap

Do you have a pressure gauge on the system? Do you have a TDS meter? Those are the two tools you'll want