Why I see weak security(WPA) when I am already setup under WPA2?

For the weak security warning to not appear see if your router has a WPA2 AES setting -- in some routers the AES option is in a different field on the same page (i.e. WPA2 is on one field and AES in another field). Also see the following from Recommended settings for Wi-Fi routers and access points - Apple Support

Security

 Set to WPA3 Personal for better security

      Set to WPA2/WPA3 Transitional for compatibility with older devices

The security setting defines the type of authentication and encryption used by your router, and the level of privacy protection for data transmitted over its network. Whichever setting you choose, always set a strong password for joining the network.

• WPA3 Personal is the newest, most secure protocol currently available for Wi-Fi devices. It works with all devices that support Wi-Fi 6 (802.11ax), and some older devices. 
• WPA2/WPA3 Transitional is a mixed mode that uses WPA3 Personal with devices that support that protocol, while allowing older devices to use WPA2 Personal (AES) instead.
• WPA2 Personal (AES) is appropriate when you can't use one of the more secure modes. In that case, also choose AES as the encryption or cipher type, if available.

Weak security settings to avoid on your router

Don't create or join networks that use older, deprecated security protocols. These are no longer secure, they reduce network reliability and performance, and they cause your device to show a security warning:

• WPA/WPA2 mixed modes
• WPA Personal
• WEP, including WEP Open, WEP Shared, WEP Transitional Security Network, or Dynamic WEP (WEP with 802.1X)
• TKIP, including any security setting with TKIP in the name

If your router or access point can do WPA3, then consider the setting ‘WPA2/WPA3 Transitional’. Then older devices can use WPA2, while newer devices will use WPA3.