range variable won't work: "must be a scalar or matrix

Unfortunately, it is often difficult to diagnose a Mathcad problem form just a screenshot; posting your worksheet is usually better (together with a screenshot showing your problem - it may work on somebody else's machine!).

There can be quite marked functional differences between objects that look typographically identical. What I think you have done is use the subscript button to enter i on the left hand side of the definition. What you should have used is the matrix index operator ... typing 'x' then '[' then 'i' should give you the result you want. The subscript button allows you to create a name with a subscripted part, such as x_init meaning, say, the scalar variable holding the initial value of x rather than a vector element with index init

Why do you type x[i= to get an answer? it should be x= or e.g. x[5=

Occupant,

you are right in that x= would be the simplest form to show x's value. However, x_i is a valid form and may be useful when one only wants to display, say, the first 5 values (i:=0..4) or every other value (i:=0,2..last(x)).