hi all, i need help , iam using a synthetic control group , for a policy inversion for five cases(countries) with a control group of 15 countries to each case , i set the data and declare it as a bnlanced panel , when i tried to follow stata command i come up with the error (control units: for at least one unit predictor opr_n is missing for ALL periods specified) , my variables are
id country year oex_n opr_n gdp gdpp gev eu fdi_n
any help please