This error might occur when you try to publish a browser compatible form for the second time (which was working previously for many times), then if you try to publish again and suddenly you see the error on the designer InfoPath 2007 "This form template is browser -compatible but it cannot be browser enabled on the selected site"
This blog was a great help for me but it doesn't show accurately what worked exactly with people:
What worked for me actually is the three steps:
1. stsadm -o activatefeature -filename IPFSSiteFeatures\feature.xml -force -url http://URLToYourSite
2. stsadm -o activatefeature -filename IPFSWebFeatures\feature.xml –force -url http://URLToYourSite
3. You need to activate the 'Office SharePoint Server Enterprise Site Collection features' feature on Site Collection level