I also did the same thing as you demonstrated.
the result is still the same.
there is an error message like the following:
"You can not add a Sub-Domain here. Please use the subdomain manager to add Sub-Domains."
Anyway, I installed sentora panel using the same domain, more precisely is panel.domain.software.
Then I tried to add a sub domain test.domain.software, then I get the error message as mentioned above.
the result is still the same.
there is an error message like the following:
"You can not add a Sub-Domain here. Please use the subdomain manager to add Sub-Domains."
Anyway, I installed sentora panel using the same domain, more precisely is panel.domain.software.
Then I tried to add a sub domain test.domain.software, then I get the error message as mentioned above.