Note that you need to check if Crystal Report for Enterprise 2016 support specific IBM DB2 version and for this you need to check product compatibility guide for development.
Are you using Universe developed on IBM DB2 then you need to ensure that you are using correct version of drivers JDBC/ODBC in IDT to create a Relational connection. You also need to ensure that connection is also published to SAP BI repository. If your version of DB2 and Crystal Reports is ok, I would recommend you follow below steps:
First steps is to start, you need to create a System DSN in 64-bit ODBC data source administrator using IBM DB2 driver.
Next is to create a Relational connection using IDT (cns) using above drivers and published to BI platform
The to create a report, you need to open Crystal Report for Enterprise and log into BI server → selected new data source connection → browse repository → select data source type (relational connection) → select the above created connection-> drag-n drop required table/columns → save it to BI platform
Check this link, it may help −