What exactly is a "web" button?
Send Mail is not compatible with IWP unless you use the SMTP option.
You can use the compatibility drop down in the script editor to check and see what script steps are compatible with IWP and which are not.
Seems strange that so many items don't work or don't work well in IWP.
Which is why it was replaced with WebDirect via FileMaker Server with the release of FileMaker 13. While that option requires Server 13, it produces a user interface in most web browsers that is much closer to the typical FileMaker Pro user interface.