I'm relatively new to Stata so I am hoping that this error is a quick fix.

I've been trying to run a Mann Whitney U test on data about whether administering a certain drug to patients affects their length of stay in the hospital. I have one column labeled GROUP (which has either "Drug" or "No Drug" as an entry) and another column labeled LOS (length of stay in the hospital, measured in hours).

So far I've tried:
ranksum LOS, by (GROUP) exact
ranksum LOS, by (GROUP)
ranksum GROUP, by (LOS) exact
ranksum GROUP, by (LOS)

No matter which iteration I try, I keep getting the same "no observations found" message.

If anyone has dealt with something similar or knows a workaround, I would appreciate it so much!