Hello everybody, I'm a beginner of Stata and I'm already facing the first issues.
I have a panel data where my dependent variable (average age by municipality, called "old" in my panel data) was recorded in 2011, on the other hand my "treatment" variable (housing prices) belongs to 2018. I simply want to regress the 2011 average age on the housing prices in 2018. Of course there are a lot of missing observations, but I thought that Stata could drop them automatically. However, when I send the command

regress housing old
I get the error r(2000) no observations.

So my question is: how do I tell Stata to match the 2011 data with the corresponding 2018 data of the same municipality?

Thank you in advance.