Hi,
Without getting too deeply into it, what is "Phi"? - first row after "Given"

Hi,

It is the heavyside step function (to switch functions).. but I don't think that's the problem..

Yes, that is precisely were I think the problem is - although I don't have proof at this time. By using the Heaviside Function, though, you are basically adding another equation to the mix which would require new boundary conditions. Odesolve likes continuous functions.

Also, what is N.i(t)? I've overlooked it before.

Beside the unknown function N.i(t), which I replaced by constant 1, I spotted some other errors:
Third equation: C.m(t) and C.s(t) should be replaced by C.m and C.s
Last equation: There is a typo between the last parenthesis and the P.N. There is a tiny decimal point inbetween which would be interporeted as multiplication by 0 - not intended, I guess.

Nevertheless the system will still fail with (as usual in solve blocks in general) a not much helpful error message.

I don't think that the Heaviside function will cause the problem. The factor phi is zero anyway so it would have no effect. If you replace it by any constant, the system will still fail.

@rmix22... thanks for ur response!!yes.. the error is not with the heavy side function..I kind of figured out the error yesterday and the system works now...

So what was the problem?
You may consider posting the file here for reference

its exactly the same as you mentioned.. with change from dynamic to fixed variable and the multiplication dot...
but then the system is not completely correct with its calculations.. I have to check with the math part anyways...