What am i doing wrong, or is my camera not good enough?

The camera may be some of the problem. However, you do not need an expensive camera to do stock photography. You need to have some skills. Looking at your first photo I noticed: the composition is not good, the shadows are underexposed, the focus is off, the sky has noise and there are artifacts.

Keep watching composition videos. Photograph in RAW if you can. Do post-processing!!!!!! Even with an expensive camera these issues are needed.

Most of all keep trying and keep learning.

Your first:

Whatever that is, it does not look right. Looking into the histogram, I see a strange behaviour, with at least the blacks missing.

By tweaking your exposure by lifting the shadows and adding blacks and by correcting the chromatic aberration (see screenshot), the picture may pass.

Second:

Check the histogram:

Blacks (left) and highlights (right) are missing, which gives us this flat, low contrast image. The picture could also be a lot sharper.

Your third: It is a nice picture. You have blown-out highlights and noise is a problem. You could crop differently, to get rid of the annoying details in front and to the right, who take away the look from the dog.

y

Your last:

There are again no blacks in your histogram:

There is a high noise level, the white car with the number plate and the logo is a property issue. The people will need a model release. The number plate on the first car may also be an issue.

The picture is also relatively soft in focus.

What was the refusal reason?

Any camera is good enough for stock, but some are better. So phone cameras, because of their small sensor and high resolution, normally produce more artefacts that lead to a refusal.

Best are full frame sensor cameras (or better) with high-quality lenses.