Question:

How to extract the data into the workflow log (Trx SWI1), by custom report?

Into Log (View Workflow Chronicle), including details of the Workflow-head, I see the object name (the number of Employee trip) and the agent.

Into the View Workflow Agents I see agent, date and time.

What are the tables or call function to extract this information?

Solution:

Extract the Work item ID from table SWWWIHEAD.

Then used the call function SWP_WORKFLOW_STEP_LOG_READ (entry with Work item ID ) to extract agent, date and time of a specific task single-step (in this way the task TS20000118).
 

Question:

When I try to get a workflow log it says,
"Workflow log is not available".

Solution:

You access the log with transaction SWI1

It could be that the log table entries were somehow removed, or maybe you don't have sufficient authorizations (check tr SU53, after getting the message).

or

You goto SWDD transaction or SWUS transaction and goto the execution screen. 

Now you will have 4 Tabs like Input Data, Ad Hoc Agents, Deadline Data and Outcome Tabs.

In the outcome Tab you can check the executed workflows select the latest entry and click on the workflow log icon.

Hope this would be the easy way and check it if you are executing the workflow through SWDD transaction.

Notes:

Also, check with your basis team at what interval the items get archived, you can check this yourself with transaction SARA and with object WORKITEM.

Get help for your SAP WF problems 
Do you have a SAP WorkFlow Question?

SAP Books
SAP Workflow Certification, Interview Questions and Configuration Reference Books

SAP WorkFlow Tips
SAP Business WorkFlow Tips and WF Discussion Forum

Best regards,
SAP Basis, ABAP Programming and Other IMG Stuff
http://www.erpgreat.com