Issue:- "Enable this Form to be filled out by using a browser" checkbox is grayed out while publishing an InfoPath Form.
Even if we try to forcefully Activate the Enterprise Site Collection Feature using stsadm, it does not resolve the issue.
By default when you Browse to the main Site Collection -> View All Site Contents. You should be able to see "Form Templates" by default if the Site Collection "Feature Office SharePoint Server Enterprise Site Features" is Activated.
If the Form Templates is not visible even if "Office SharePoint Server Enterprise Site Features" is Activated then follow the below steps
1. Open the Main Site Collection in SharePoint Designer.
2. Select the root URL and Right Click -> NEW -> SharePoint Content -> Select Document Library -> Select Form Library -> Under options type the name as "FormServerTemplates". Click on OK.
3. The Library will be created. Right click on "FormServerTemplates" -> Select Properties -> On settings Tab change the name to "Form Templates" and uncheck the option "Use a template for new documents"
4. Click OK. Now you will see the "Form Templates" on the view all site content page.
5. Deactivate and Activate "Office SharePoint Server Enterprise Site Features" once again and now You should be able to publish the InfoPath form with "Enable this Form to be filled out by using a browser" checkbox Enabled.