When I click add new account in online account panel I got blank as below:

Given your clues... not easy to answer but as a start...
check that you can see the accounts service is running...
and that you have kaccounts-providers and kaccounts-integration installed

abury wrote:Given your clues... not easy to answer but as a start...
check that you can see the accounts service is running...
and that you have kaccounts-providers and kaccounts-integration installed

Sorry fo the lack of imfomation. I copied the text here and seems it lost a part of it.
I am using latest Fedora 32 plasma version. After I installed kaccounts-providers and kaccounts-integration I can see google and owncloud login in add new accounts now (and only this two) however it will stuck forever at blank page with name as Web authentication with google

Update:
Later I tried the add new account (google) again and I just randomly clicked and pressed enter a lot at th blank page and somehow it suddenly tell me it succeeded..... However the account only serves for Gdrive and youtube. I don't use youtube and I use rclone to mount google drive so this is kind of meaningless and what I am expected is to have my mail and calendar synced. (I am also expecting to sync my another account which is exchange protocol)

https://forum.kde.org/viewtopic.php?f=309&t=163200#p427777
https://support.google.com/accounts/thread/24852814?hl=en&msgid=25773797
These links seem to indicate that there may be a problem on the Google side...
Also, when looking at available APT packages, I see gnome-online-accounts listed as well
install that and see what transpires
if there really are upstream problems, then it can always be removed...
(And given that you're on Fedora... check with their forum to see if they have details specific to their distro)

It is odd but I kept spamming Right Click -> Go Back on the blank screen and pages loaded.