Well, these are our support policies:
https://toolset.com/toolset-support-policy/
Please read them, to understand why we cannot offer custom code of this kind.
The request of code you have is not related to Toolset.
1. If there are classes or a certification you offer I'd be more than happy to learn as much as possible from you, and potentially become a contractor myself down the road.
In fact we have trainings for aspiring contractors, but you first would need to qualify as one.
That is only possible if your profile qualifies, which I am not sure - but you can see it in your profile when you open the main profile page, if you are qualified for it you will see a message inviting you to participate
See also https://toolset.com/2017/11/toolset-contractors-are-waiting-for-your-projects/#comment-329425
I hope you understand that we cannot train you here in this support forum so later we can send other users to consult you as a contractor.
2. I am actually asking since a while for exactly what you request (paid training for people aspiring to learn more about generic coding, and WordPress) - I believe not only there is a great interest but also (last but not least) a great opportunity for the community if we would proactively train people in custom coding and other things, but - I am not at a position to decide this, and it is not currently the path Toolset goes as a WordPress plugin, or it's support.
I will mention the requirement again to our management and you can always as well ask for changes (even in support) at https://toolset.com/home/contact-us/suggest-a-new-feature-for-toolset/
3. the toolset function 'update_post_meta' is not being recognized for some reason
update_post_meta is not a Toolset function, as much I would like it to be.
The code you use is pure PHP and WordPress API, of which we cannot help here - we can explain and instruct but not debug such custom code or write it for you.
Toolset API has no update_post_meta function.
4. Call to undefined function update_post_meta() in C:\wamp64\www\LDPOS\wp-content\toolset-customizations\markpaid.php:24
That means you call the update_post_meta too soon or at a moment where the core files of WordPress holding that function are not loaded
I suggest to consult the excellent Stackoverflow article I shared with you earlier that shows precisely how to construct such a custom logic where you call a PHP method with JS (basically this is AJAX).
This is not a simple topic, but it will help you to understand how this works and how to set it up in a simple example, which you can then adapt to your custom situation.
I apologies that I cannot be of better help here, at this moment.
Please let me know if you have questions related to Toolset and it's plugins/API, with which I can help much "more in depth".
Thank you for understanding