Good afternoon!

I regress the model:

GDP = β0 + β1ERR1 + β2ERR2 + ơ1Pop+ ơ2Surf + ơ3ExGS + ơ4ExpEd + ơ5RIR + u

My main focus is ERR variable which is categorical (exchange rate regime of the country). The base level is floating regime, ERR1 is pegged regime and ERR2 is residual regime.
Thus, I need to test significance of these variable (ERR). I try to use command ttest, but Stata shows me mistake.

ttest GDP, by(ERR)

more than 2 groups found, only 2 allowed

I tried to understand more appropriate test on https://stats.idre.ucla.edu/other/mult-pkg/whatstat/, but the table is not clear.

Could you recommend which test to use for whole model to asses it and test separately for categorical variable?
I need to understand significance of the model and of the variables. So I more lean to F test for whole model and t and p tests for variables. Thank you!