Occasionally, a publishing job from ArcGIS Pro to ArcGIS Online can get "stuck". This can happen for a number of reasons, one of which being the C drive becomes full. To fix this issue, navigate to C:\Users\<username>\AppData\Local\ESRI\ArcGISPro\Staging\SharingProcesses\ and delete all data in that folder. If data cannot be deleted, force close ArcGIS Pro and try again. If that still does not work, restart the computer and delete the data in the above folder.