Hi there!
I wanted to set antenna gain for my hap ac3 to 0 but I get the following error:
“失败:没有t allowed to set antenna gain below 5"
and I noticed that I can’t choose any country rather than Azerbaijan.
I’m not from Azerbaijan but it seems like my device is imported from there.

I want to know if I could remove this country lock by reinstalling RouterOS using Netinstall.
Thank you.

Yes

It is likely that you won't be able to set antena gain to less than 5 in any case. Modern ROS versions have hardcoded minimum antenna gains for devices with built-in antennae and ROS doesn't allow to set this property to value lower than the hard coded minimum. And this has nothing to do with selection of country.

and I noticed that I can’t choose any country rather than Azerbaijan.

One can only change region, if all data entered is within range. (including antenna gain, which is not visible in the newer ROS versions (like 6.48 onwards))
e.g. having changed the antenna gain to a lower than the current mandatory minimum value in a prevoius old config , will disable any change in Country.
So set this antenna gain to at least the minimum value, reported to you as being 5. (minimum which is set by MT to be the physical characteristic of the antenna delivered,) and try changing country again.

This is for the "MT wifi classic drivers". Country Lock? Who did this ???
Wifiwave2 can be different. Didn't test this yet.

However I know that wifiwave2 tends to allow 3 more dBm because of TPC handling.

While checking there is a hughe difference in allowed EIRP between classic wifi in 6.49.6 and wifiwave2 in ROS 7.5beta8, for region Azerbaijan.
Could not test classic on hAPac3 now, because classic<->wifiwave2 switch requires install/remove of the wifiwave2 package AFAIK. Inconvenient right now.
But should be the same for hAPac2 and hAPac3, or did it change with the ROS version?
The wifiwave2 EIRP limits are as good as those for the rest of Europe. The classic ones here are lower (18dBm)