That particular error message is seen when DB2 database provided in the connection string of relational connection is incorrect. For DB2 native connectivity, DB2 database name should be provided as connection string. Note that the database must be cataloged on the server hosting the Integration Service for this to work.
Refer to following KB article for necessary configuration: https://knowledge.informatica.com/s/article/19254
You must have an odbc connection on your machine which hosts the client tools but you also need to have an odbc connection on the machine which runs the Integration Service.
The odbc connection which you use for your client tools must be configured to use 32 bit and can be either user dsn or system dsn.
The odbc connection on the server must be configured as a system dsn since very likely the user which runs the informatica services differs from the user with which you logon to the server to configure the odbc connection.
Please also note that an ODBC connection to a DB2 server is not allowed with PowerCenter. You have to use a DB2 CLI client (or Db2 Connect client, but there are a few points to observe, please look at the respective Support Statements).
ODBC to DB2 is allowed in ICS/IICS/IDQ, but not in PowerCenter. Or at least that has been the case for >15 years.