Hello Nikhil,
Questionnaire tab will automatically enabled once you assign Questionnaire to Service Order at customizing Hearder level.
- Path: IMG-->CRM-->Transactions-->Define Transaction types
- Select "Service Process" at Assignment of Business Transaction Categories
- Double click on "Customizing Header" from dialog structure
- Enter the Questionnaire you defined
- Save
Now, go and check "Questionnaire" tab will be enabled.
Thanks
Preethi.