To assign a value to a name, use the syntax name := value, not just name = value.
Maple provides "literal subscripts" that are in essence, Atomic variables, and "indexed" subscripts that are essentially table values. If your Maple is set to form indexed variables when you subscript, then note that you should not assign to a name such as y if you intend assigning to a subscripted version of y. It is because of this "gotcha" that the literal subscript was made the default in Maple. You can see all such atomic variables if you select "Atomic Variables" in the View menu. All atomic variables show up in a maroon color.
The Maple syntax y creates an indexed variable. If you had assigned to y, you would then not want to assign to y itself. The name y pretty-prints as a subscripted y, so it can easily be confused visually with the Atomic version.
Assuming that your Maple is set up in its default mode, your subscripted variables will be Atomic, and you can ignore the complications referred to above.
I didn't find where y_sub_one was assigned a value. Hence, in the plot command, that quantity is unknown to Maple.
Finaly, in the plot command, the square brackets around what you want to graph are superfluous. Square brackets represent a list. You can graph what's in a 1-element list, but making a list for the graph of a single expression isn't necessary.
And, by the way, the error message is telling you precisely what the problem is. It's saying that y contains no t, the variable that the plot command is looking for.