No numeric data plot x axis

I am trying to solve this:

syms X L

X = 0:L/16:L;

Y = 1 + 2*X;

plot (X,Y);

And appears the error:

Error using plot

Data must be numeric, datetime, duration or an array convertible to double.

[Walter Roberson]

Duplicate of https://www.mathworks.com/matlabcentral/answers/805161-non-numeric-data-x-plot?s_tid=srchtitle which has an answer

Answer 1

[Tiago Araujo]

https://www.mathworks.com/matlabcentral/answers/805161-non-numeric-data-x-plot?s_tid=srchtitle#comment_1466666