Hi, If you can include the error messages in your post, this will help us tell you what they mean.

DAE stands for Differential Algebraic Equation. There's more information available about them online. The index of a DAE is how many times the system has to be differentiated to turn into a set of regular differential equations. In this case NDSolve can handle order 1 DAEs.

The notebook attached has many different parts. I would begin by producing the simplest possible example of the problem and removing extraneous material from what is causing the issue. The issue is with NDSolve, so I'd begin by finding what equations were given to NDSolve.