Many thanks to the AppSheet support team for their valuable: cooperation, help, time spent doing tests for solving this case and for their quick response.
The final answer to this question was:
"As per the updates from our specialist team, we would like to inform you that AppSheet no longer supports adding new "On-Premise database" connections. If you have already configured the On-premise database, it would continue to work, but any new connection won't work. In case you want to use an SQL server, you may try using a cloud hosted server".
I hope that in the future, Google AppSheet can consider including connections to local databases and not just in the cloud, so that customers can have hybrid solutions.
Sincerely
Alejandro Uribe
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