As a new seller, you will have a limit on the number and value of your listings.
For example, you may only be allowed to list 10 items OR $500 in value.
Which could be one $500 item or ten $50 items -- or ten $10 items, which is safest when you are learning.
And it is always a good idea to do some buying to learn how eBay works from that side and also to build up some feedback, since low feedback sellers are scam magnets.
That being said.....
Are you using a phone to list? It is much easier to see what you are doing on a laptop or desktop.
Are you using the default and terrible Simplified Sell Your Item form? The clearer and more detailed Advanced/Business form is linked at the top right of the Simplified.
Have you tried turning it off and then on again? Sometimes computers get in a loop. Let the computer gnome have a coffee break, he may be more cooperative after.
It may be your browser. I use Firefox because a few years ago Safari stopped accepting eBay uploads. Others have problems with FF and use Chrome. I blame the gnomes.
Be aware that even when your listing uploads, it may take up to 24 hours to "index" and show on the site. This is even more likely with new sellers and with listings in high value/high fraud categories like electronics or horse tack.
