Found on a WOW Facebook thread and seems to have worked for many. Hope it helps:
Close out Battle net completely (use Task manager to ensure if need be)
Navigate to "system partition (C:); Enable “Hidden items”
Open Program Data
Delete “Battle.net” folder and restart your Blizzard desktop client.
Same! Stuck at 0% for over an hour. I can log in using the World of Warcraft app, but don’t use the Battle.net app. All my addons are DOA as well! Thanks for that! Not playing until this is fixed. Not buying Dragonflight until this is fixed.
Similar issues across other games also. If you have the stuck initializing error and try and do a clean install of battle.net it will also hang up and not update.