I dont know why this keep happening, can someone help? I would be very grateful...
There appears to be nothing wrong. Your evaluation sol= gives an answer that tells you that sol is a function. You can plot it versus time; or you can plot the function versus its derivative as Valery showed. What you will NOT get, but maybe expected, is that sol= would result in an expression of what the function looks like. Mathcad Prime does not have the capability to symbolically solve differential equations. If you need that, use Mathcad 15 (not Prime) and look here.
It could look like this:
Or, with more symbolics, like this:
Or you use the symbolics in Prime together with (your) knowledge of Laplace transforms in solving ODE's. Look at Ttokoro's post below. Note that you get neater symbolic results if you use exact numbers instead of reals: 3/10 instead of 0.3 and 1/100 instead of 0.01)
q(t) [C] of this electric circuit shows the same result.