Unable to extract a value from a matrix

As @ttokoro  has illustrated, you should first assign the function to a variable and then evaluate it. Once you've done this, you will be able to access the elements:

What you defined is not a matrix or vector but a range. A range can be seen as sort of implicit loop and not as a collection of values. So you can't access individual values like you can do with a matrix/vector.

Its a pity that in Prime there is no difference between the display of a vector and that of a range - they look the same and this always has resulted in some confusion. You sure are  not the first to fall into this trap.

Ranges should be used just for three purposes:
.) as abscissa values for plotting a function
.) in a prgram when using a for-loop
.) to index the elements of a vector or matrix

What @ttokoro and @Perez  had shown is an undocumented trick to turn a range into a vector by so called inline evaluation (evaluation by adding an equal sign at the end of a definition)

The screenshot is of Mathcad 15 and as you see in this version there was a little difference between the display of a range and a vector. Nonetheless ranges got confused with vectors in this version, too.

Using an undocumented trick may be a bit dangerous as it can change its behaviour without any notice in future versions. But I alsi use this trick ever so often because its so convenient 😉

@ttokoro also provided a more "legal" way to create a valid vector and there sure are different ways to create a valid vector. If you search the forum you will find different utility functions to create a vector, either by providing start, second and last value as with a range definition, or start and end value and number of elements or start, step width and end value, ...