It says: "There are still screenshot uploads in progress." when submit a new build

I faced the same problem. For me, the solution was to 're-convert' the image. For example, I had an image in a format specific to the store. I went to a website that converts images to pixels and converted it. Then, I saved and uploaded it to the store again. After that, I reloaded the page and it worked!

I had some images that had semi-transparent backgrounds - some it would accept some it would not - I didn't need/want/intend to have the transparent backgrounds once rectified all images uploaded the very next time.

For me, it was related to transparency and alpha pixels in images. I solved this by converting the images into JPEG format.

This is because I use an external tool to generate screenshots for the specific size required on the store, which exports images in PNG format.

Although the images do not have any transparency because I can see a white background, but somehow they had alpha pixels that were hard to see with the ***** eye.

For me it was simply opening a private/incognito window and doing it from there.

The same issue due to transparency. WHY DONT SHOW AN ERROR RIGHT AFTER SCREENSHOT UPLOAD??????

Using a private window in safari browser works!

Choose File > New Private Window

Nothing else worked for me until I opened a new private window in safari and then they uploaded just fine.