You could try the following:
- Check if your e-mail has been verified (Account Settings -> E-mail adresses)
- Disable your VPN for the uploading process
- Use a different browser
- Try disabling your antivirus (at your own discretion and with that specific tab being open to try the upload)
- Try restarting your internet connection (On your decive and/or router) or your device entirely
- Wait for some hours or a day, maybe there simply is an issue with the server right now
If none of that works, it may be a more specific issue in regards to your internet connection / location. In that case, maybe someone knows a bit more about that.