In Qlik Sense It's not possible to preview tables and fields in a SQL database if the databas name contains a dot "." character. Dot characters in field or table names can also be problematic.
Environment:
Qlik Sense Enterprise on Windows
Cause:
Dot in database names is generally considered bad practice and should be avoided:
Resolution:
You can try switching Sense to Legacy mode and manually constructing the connection string: See Disabling standard mode
However there is no guarantee that will help, the best solution is absolutely to follow best practices and avoid dot in any database or table names.