You need to grant the Zabbix Agent user access to the SQL Server instance.
nodata(/YourHost/perf_instance["SQLServer:Databases(_Total)\Data File(s) Size (KB)",_Total],30m)=1 and last(/YourHost/perf_instance[...])<>0
Note: Replace [NT AUTHORITY\SYSTEM] with the specific user running the Zabbix Agent service if different.
# /etc/zabbix/zabbix_agentd.d/mssql.conf
You need to grant the Zabbix Agent user access to the SQL Server instance.
nodata(/YourHost/perf_instance["SQLServer:Databases(_Total)\Data File(s) Size (KB)",_Total],30m)=1 and last(/YourHost/perf_instance[...])<>0
Note: Replace [NT AUTHORITY\SYSTEM] with the specific user running the Zabbix Agent service if different.
# /etc/zabbix/zabbix_agentd.d/mssql.conf