Whenever possible attach the worksheets, not just pictures. Its hard to debug a picture.
Even though that in your case the reason for the error seems to be clear even from the picture. Simply delete the equal sign after the definition of the function v(t).
You can't evaluate a function definition, you just can use it.
If you then type f(t)= in a separate region you will see what you expect.
BTW, its not necessary to define the range t before the function definition as it has no effect there anyway. It suffices if you define it in front of the plot you a trying to create.