A/C Only Works at Highway Speeds

So I have owned this 2001 Suburban since last October.

To my knowledge the A/C worked fine and had been working fine. However, the drain is clogged and I could never find where to blow it out.

Since the A/C is not working but when I am at highway speeds and have it on after about 10 Minutes the dash makes the suction sound and then the A/C work and she blows cold.

However, once out of highway speeds and in city drive it stops working.

I thought I'd try charging the system but when I hooked up the R-134a setup I have the meter read in the RED which means alert. It did not read low or normal.

Looking to see if anyone has suggestions to the problem.

I did confirm that at idle if you push the A/C on and off the clutch does not engage. However, like I said at highway speeds it works just fine.