To check that the connection to the server is valid:
sqoop list-tables --connect "jdbc:sqlserver://<server_ip>:1433;database=<database_name>"
--username <user_name>
--password <password>
Before doing this it is recommended to check if SqlServer's configuration allows remote access to 1433 port.
Open Configuration Manager => SQL Server Network configuration => Protocols for MSSQLSERVER and check that Protocol is enabled and the needed IP and port is enabled and is active.