I have had rejected this image.
Because of technical issues. But I dont know what it means.
I am still inexperienced in this field.
Can anyone help me or give me advice for the next time?
I think you have cropped the pelican too closely to the edges. There is not enough room for the feet and the bird's tail and beak. If someone wanted to crop this image, there is no room for cropping to the size and proportion they would want. You need to give more room around your subject. Also, I think a very small adjustment in the white balance. The image is just a little cold/blue looking. So a slight increase in colour temp would help.
You should read the Adobe guide to taking better images:
Lovely bird. I believe the reason for rejection was for the white balance issue. Zoom up to 100% or probably a little more and take a close look at the top of the wing, and the black and white feathers in the area of the breast and shoulders. You will notice it is a little blue. You can adjust this by increasing the temperature slide a little, and resubmit.
While I do not think composition is the reason for rejection, you still need to note what Ricky says for future framing. It is also important.
In my opinion you've oversharpened some areas. Unfortunately, the wrong areas. The eye seems to be a bit out of focus while the chest feathers are hyper-sharpened. There appears to be artifacts in the image where the water is, my guess is this is due to oversharpening.
Better luck next time,
my 2 cents here, let the subject "breathe" and - this is just a personal opinion - give your photo a bit more of color, let it live. 🙂
I agree, is is too closely cropped but I agree with @MatHayward that the image is not sharp.
When doing wildlife photography, it is essential that the eyes are in focus.
You can correct some focusing mistakes in Lightroom.