My posts get deleted because they don't meet the artistic quality standards. For me, they seem to have a very high quality. I don't know what I'm doing wrong.
The size: 3000X3000
Resolution: 300
No whitespots
Smooth lines
These are the artistic quality standards! Can someone tell me what I'm doing wrong? Can someone help me to meet them? I'm really frustrated about it.

Can you provide the images in question? We can't exactly tell you what was wrong if we can't see it...

"Does not meet minimum quality standards. (Artistic)" generally means that artistic ability is in question, not the viewing/file quality of the image.
Looking at post #2514103... you kind of have some ZUN-esque sausage fingers going on. The width of the digits varies in a way that doesn't really make sense, and separately they kind of lack the form one would expect from fingers, the nails appear to just come straight out of the center of her fingertips. The lighting is odd in a few places (such as the left side of the image where her breast meets her shoulder), suggesting there's something like a light source near her armpit. Yet, that light arbitrarily turns into shadow once you move pasts the breasts and onto the lower arm. post #2508289 and post #2506619 also have eye shapes that make absolutely no sense. Pac-Man/Mickey-Mouse style eyes that basically have a pie-slice taken out are fine, but Cookie outright has an eye with a square corner, and Rosie pupils with a similarly strange shape.

The background looks realistic btw.

... realistic for what? It's an indistinct rust-colored texture.

Separately, it would be a good idea for you to upload somewhere else, where you're in control of your uploads, rather than e6. Gelbooru, a site very similar to e621, has this to say about artists uploading their own work.

Uploading One's Own Art is a Good Way to Get Your Feelings Hurt

Posting one's own art is severely looked down upon, and there are two reasons behind this:

Firstly, artists tend to be biased when it comes to judging the quality of their own work. What looks good to you may be regarded as mediocre to Gelbooru moderators.
Secondly, more suitable sites to upload your own art to include Deviantart and Pixiv.

Admins and moderators are especially harsh on self-posting artists, lashing out penalties without remorse. The surest way to feature your art on Gelbooru is to show immense talent and skill elsewhere, and with that, captivate some fans on the other side of the Internet into uploading your art.

Art quality is very little about the specs of your style, and I'd say 3kx3k is quite absurd, and by no means a minimum. Looking at your uploads, you still need to improve upon your illustration skills. You could stand to improve your sense of anatomy, as well as shading. I must say, it doesn't help that most of your pieces are all in the same lackluster pose - that certainly detracts from the "quality".

As Lafucadio was saying, you'd probably be best posting elsewhere. You can get feedback and improve upon your art much more in other communities. E621 simply isn't built for that. Also, you'll probably get more interaction in general elsewhere, and it's better for networking. FurAffinity or DeviantArt will probably be a better fit for you, for a number of reasons.

I hope this feedback doesn't deter you from creating! I hope you have fun in your studies. Cheers!