There is a lot on this forum about having enough reputation to post to old posts, use certain tags etc. Even though I don't post very often, my posts are definitely not spam. It is quite frustrating (and unnecessary) having to create new posts that totally relates to something that already exists. Can someone please look at my posts and add me as a "trusted poster" or equivalent? Also, what is this about "You can only post once every 20 minutes"? I had to wait 20 minutes to add this post. What if I have many questions?
[not] having enough reputation to post to old posts
Questions will (on occasion) become something known as protected. This is usually a result of a question:
- being very popular
- receiving lots of low-quality answers
- receiving lots of non-answers (me too, thanks, spam)
All protection does is limit new users (defined as under 10 reputation) from answering them. You can earn ten reputation on this site very quickly - that's just receiving two upvotes on a question or one upvote on an answer. You get these by, of course, writing good questions and answers.
[not able to] use certain tags
If you have under 300 reputation, you are not allowed to create tags. This is intentional, as tags are rather important to allow users to find questions and to offer help. A new tag should only be created if it's absolutely necessary. The 300 reputation limit gives users some time to realize how the site works, and to think about whether a tag is necessary. While we still do get weird tags on occasion, these are usually rare thanks to this limit.
Otherwise, certain tags (like installation) are what we call "blacklisted," meaning they can not be used. Blacklisting is a heavy-handed operation and is only done for good reason. Find and use a better tag in this case.
Also, what is this about "You can only post once every 20 minutes"? I had to wait 20 minutes to add this post.
Consider this from the site's perspective. A spammer comes on, and makes a ton of spam posts very quickly, flooding the site with spam. Legitimate questions are dropped to the bottom, and nobody can get the answers they need. Moderators and high-reputation users need to clean up the site. It's a mess.
So, this rate limit exists until you've earned your ten reputation. There's no overriding this, no sneaky ways around it.
As mentioned in the comments, this is also a way to ensure that the posts you write are thought out, thorough, and useful to the site. If people post garbage, nobody will be able to help.
With that out of the way, let's look at you in particular.
It appears that your initial goal was to post this question inside of this (existing) question. By doing this, you likely ran into protection after making that post. This happened because users like you did the exact same thing.
It is very very important to understand that Ask Ubuntu is not a forum. We do not like long-winded threads where people ask each other questions and go three pages before someone finds a solution. Here, you ask a well-formed question and receive a well-formed answer. Chatter is intentionally minimized and thrown out of the way, as the goal of this site is to get users help.
Now, due to the nature of some tech problems, this isn't always the easiest. We need to debug posts. This is why comments are a thing -- they allow for other users to ask for clarification and for you to respond in kind. If things become long-winded, comment threads will often be moved to chat where users will help you diagnose problems live. This is all part of the "flow" of this site - you come here and you get help directly. This is why "accepting" is a thing, and why votes are there. They all directly relate to getting users the help they need as fast as possible.
That said, I would very strongly recommend you read our site tour before using this site. It'll explain quickly how to best use this site, as well as offer links for more in-depth learning. The tour is your friend.