Require.js not imported/loaded/available to HTMLSnippet widget or index.html
Since mendix uses AMD pattern for widgets, (that architecture has require function as framework default) so it must be getting the reference of that!
When you run the app, open the browser and go to browser console, and simply type console.log(require) or console.info(require) see what object you get.
My doubt is, it is referencing the dojo frameworks require function. but i could be wrong.
then again i am not sure what are you accomplishing by adding requirejs libraries?