Hi Tun-Ching Yang,
You can try the following two methods:
1) In the lower right corner of the Toolbox you will find a "view all/customize" buttion. This will open a window and checked applications wil show up in the Toolbox.
2) If approach 1 does not work -- complete exit the Ansys Mechanical Program-- click on the Start Button and enter the command %appdata%-- this will open a file manager window-- change folders to Ansys-- look for the folder named "v followed by version number", for example for 2023R1 - v231. Then rename it to an arbitrary name. Ansys Mechanical will recreate this folder the first time you restart Ansys Mechanical.
If both these approaches don't work, you can also try restarting the license server.
You might find the following video useful:
Ansys Licensing: Managing Activations - YouTube