Since you've posted this question within the System Archtitect Essentials I course forum, I'm going to take a wild guess that by "primary key value" you are referring to the Case ID (.pyID) property found in the case header.
First thing to note is that the Case ID is automatically generated the moment the case is created, regardless of the UI. The Case Header is simply displaying the property value for .pyID.
The Case Header is one of the many sections referenced in the standard Perform harness rule record.
You can use the UI inspector, hover over the case ID (e.g. "C-14") and open the standard section rule record named pyCaseHeader.
Create a copy of this rule into you application RuleSet: Click on Save As, and when the SaveAs dialog appears, don't change the name of the rule or the AppliesTo class reference (leave as Work-), simply change RuleSet reference to something in your application (e.g. HRServices:01-01-01).
Delete the cell that references .pyID, Save your new rule, and test by creating an new case. You will no longer see the reference in the header, but if you use the Clipboard viewer, and review the contents of pyWorkPage, you will see that the .pyID property has been intantiated and has value (e.g. "C-14).
Below are screenshots of this process, although references exact RuleSet is different - in your case for SAE I course, I'd place the new pyCaseHeader section in HRServices ruleset:
Thank you. How can I save this caseID as a primary key value in my Data Table? should I mention this in a property-set(activity) or in section itself( Text Input->General->properties as pyWorkPage.pyID)?
There seems to be some confusion. Based on your response I would conclude my first reply is not what you are looking for. "Primary Key" in a Data Table is a completely different subject than Case ID, so my previous assumption was incorrect.
Because your question involves a design question, it requires a more nuanced answer. Based on your particular example I suggest that you consider reaching out to Pega Consulting http://www.pega.com/services/consulting for further assistance.
The particular forum you have posted in is dedicated to Pega Academy, System Architect Essentials I (7.1.6) course topics only.
Typically, Data Tables (Pega 7.1.6) are created for the purpose of storing small sets of read-only data that does not change or need to be updated frequently.
IMPORTANT UPGRADE NOTE: In Pega 7.1.7 and forward, Data tables have been replaced by a new local data storage format. You can upgrade old-style data tables to use the new local data storage format, but the new format is not compatible with earlier versions of Pega 7 in which data tables could be created.
You can still edit legacy data tables by using the old data table functionality. However, to create new data tables, you must use the Data Explorer and Data Designer.