I have several questions, can someone please answer me?
1- Can I use it on the main domain, myweb.com, and not on a subdomain?
2- Can I migrate from one VPS to another?
3- Are there any plugins that I can sell within the system that are unlocked when I buy something?
I look forward to your response.
Yes, but it’s better for several reasons to use www than the main domain. To the user looking at the url in their browser the bare domain and www look the same
Yes. Backup on the old one and restore on the new one.
You can use Discourse Subscriptions
Any tutorial?
Search is your friend here. If you come up blank, let us know what the gaps are but you need to do your homework first. 
I was installing it and it gave me an error
Could you share the error?