As johanpw already said, simply drop using J25, and Jump to J3.
Regarding the Uncategorized:
You will need at least 1 category, where your articles should belong.
The name "Uncategorized" is a conventional name for a general category, where you can assign any number/kind of articles.
What happens if you delete it?
Well, first of all, you won't be able to completely delete it, if it has assigned articles. Otherwise, you are ok to delete it.
What about the error in the front-end of the site?
Most likely, the page you were looking had a relationship with this category. It could be a Blog Menu Item assigned to display articles from that category. But with that category deleted (or trashed), then the front-end page is losing its reference to the database and it manifests this with an error.
How to delete it, without getting the error?
First make sure, any articles you really want to keep aren't belong to that category. Move them to a category you want. Then, you need to make sure that none of your menu items is linked with that category. The fact that you are deleting the category, means that you also don't want to show anything from it in the front-end. Check all your menu items, and change their settings.
After that, I think you should be ok.
Bonus: An example workflow of setting up a simple website
The uncategorized category (which you can name it anything you want), comes very handy, if for example you want to create a simple 4 pages website.
Let's say the structure of your website will be like:
Home | Company | Products | Services
For this, you would create 4 articles with the corresponding names.
So, you start creating the articles one by one in the Articles Manager (com_content component), adding your titles, typing your content, adding images. Since the website is simple, you don't need to to create a complex categorization structure. You would be just fine, if you assign all your articles in 1 main category. This could be the default uncategorized one, or you can rename it anything you want, or you can simply delete this and create another one - it really doesn't matter.
Next steps is to go in the Menus (menu manager) and create your menu items for each of your pages. You name each one the names as per your structure, select menu items of the type: Single Article, and you select the corresponding article.
Completing the above steps, will give you a simple website.