1. Not reading the articles where you submit a query or article. You have to read the magazine or online publisher's style before submitting a query. You want success and a "yes, we will take your article or story." You get that by reading the magazine and getting a feel for what they want.
2. Revise your article. When you write anything you should let it sit for a little while after you complete the article. That was a feat! You just wrote something. Before you send it in, re-read over it out loud. Make sure that it makes sense and sentences flow for easy reading. The one mistake many writers make is not revising the article before sending it out.
3. Grammar check. This is not Microsoft Word grammar check. You should read the article or story out loud. Notice where you get stuck or lose focus. Those areas need cleaning up with your tone of voice or grammar. You can use a grammar check software to help you with this area. I do. I use Grammarly and Editor. They work great but still do not substitute for reading the article out loud.
4. Aiming too high. When you want to get into publishing, do not go for "Oprah" or "Good Housekeeping" right out of the gate. Aim for lower paying publications first. Get a copy of Writers Market and select some with one dollar sign next to them to start. This helps build your clip file and portfolio. After having a few publishing credits, then aim high and submit a query to a higher paying magazine.
5. Not determining the market for the article. In a recent class I took for journalism there was one guy who loved writing about history. He wanted to submit to a high paying magazine. The teacher nudged him to look for lower paying ones first. History seemed to only have a place in certain magazines that you aimed at readers in the particular state.
While writing history has its place, there are so many writing ideas that are more marketable. Read through magazines to see what new spins you could give to an old idea. If they publish the topics then these are marketable. If you want a marketable idea, you can look at what sells in magazines. Get a feeling for your readers by reading the advertisements on a website or in a magazine. If it is listed, it sells. Come up with a query based on that information.
Let me know if you have any other questions and hope I answered yours!