The most likely cause for this is the holiday schedule. As per Apple:
App Store Connect Holiday Schedule: Dec 23xe2x80x9327
Make sure your apps are up to date and ready for the holidays. New apps and app updates will not be accepted December 23 to 27 (Pacific Time), so any releases should be submitted, approved, and scheduled in advance. Other App Store Connect and developer account features will remain available.
If you feel this isn't the cause for your issue, then the next thing to check is that you haven't forgotten to provide some info in one of the required fields. Any fields with missing data will be highlighted for your convenience.
One thing that also catches people out is the Build. Make sure you've clicked on the + symbol to choose the build you want to submit.