WebSphere Application Server - Express for iSeries offers several methods you can use to troubleshoot problems. Which method you use depends on the nature of the problem. Generally, you use a combination of these methods to determine the cause of a problem and then decide on an appropriate method for its resolution.
These resources provide general troubleshooting assistance.
See the Troubleshooting section of the WebSphere Application Server - Express for iSeries Information Center.
Read the WebSphere Application Server - Express Release Notes 
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Wrong version of i5/OS installed on your server.
WebSphere Application Server - Express runs on i5/OS versions V5R2, V5R3 or V5R4. The product can not be installed on i5/OS version prior to V5R2.
Installation fails due to "Object not found" or "Not authorized" errors.
The user profile of the user installing the product must have *ALLOBJ authority and have a value of *SECADM for special authority.