please clear your browser cache and try again. Alos please check your internet speed. Still problem then you should opt in for FTP upload instead, it is the most certain way of uploading.
Open your FTP client and enter the following details:
Host: ftp.marketplace.envato.com
Username: [your Envato Market username]
Password: [your Envato Market API key] Generate a legacy token in your profile, and use as password.
Port: 21
Click ‘Connect’ or your FTP client equivalent.
https://help.author.envato.com/hc/en-us/articles/360000518446-How-to-upload-using-FTP
Once you upload via ftp using client like FileZilla, just refresh the item submission page and you will see you files there.
How to get an API Key
You can try with other browser, try after removing browser cache or restart.
I have face this issue with google chrome browser.
Or you can trying to continue it will be uploaded.