Sometimes the name of the files can vary from one template to another. Also the tutorial you are trying to follow is an old one. A lot has changed since then in the theme system especially with the advent of Online store 2.0 themes.
I have found a solution for your problem. It is a solution that will save your work. Find in the GIF image below a screenshot of the solution:
Solution
For the “Add a Size chart button” section of the tutorial you were following, do the following:
In the Sections directory, click to open your main-product.liquid file.
Paste the code of block A just after line 314 where the code {%- when 'buy_buttons' -%} is located in the file specified above. If you cannot find the code on line 314, please look for the expression when 'buy_buttons' in the file and paste the code of block A just after the line with the expression searched for:
Block A
{% if product.options contains 'Size' %}
See Size Chart
{% endif %}
Would you kindly show me the content of size-chart.liquid in the Snippets directory and the content of your main-product.liquid file in the Sections directory?