MacBook Pro asking for WPA2 password

If you want to "extend" your network, the idea is you give the extender the SAME Network-name and the SAME password. Then you will have ONE Network like one of these:

The top model (with Ethernet between the stations) gives better performance. The bottom model burns up some of the bandwidth in Router-to-Router talking.

The initial choice of 2.4 and 5 using the same name allows your Mac to choose. It will likely choose the best connection (least garbage) which is not always the fastest or most trouble-free. So the OPTION exists for giving the two bands (2.4 and 5) Different names so that you can selectively join one or the other to get around unusual conditions.

The default WPA-2 password written on the Router should have been replaced by your daily-use WPA-2 password when you installed your original Router. So it should no longer be valid unless you do a Deep Reset (usually with a paper-clip) on your Router. (not suggesting you do that).

The range extender needs to be told it has the same network-name, and needs to be set up with WPA-2 and the same password as the main router, but in"bridge" mode, (or whatever term they use).

No complications, the Router and Extender will adjust. Then you have to tell each computer and phone to join the new network-name, if it has changed.

Almost all Routers since 802.11g will combine themselves happily. Channel selection should be set to "auto" in each Router, so they can adjust for existing conditions and competition from your neighbors.

Are you sure that there is no fault made in setting the password in the extender?

(Correct that if necessary by doing a hard reset of the extender, then putting in the password)

Indeed, WPA2 is the security level of the WiFi password. Change that password to the WiFi password you have on the main router.