Macbook can't connect to the internet via wifi extender

When you first turn on a Mac or any device on a Network, if set to get an address via DHCP, it actually gives itself a 'self assigned" [i.e., good only locally for station identification, not for real data] IP address.

But if any Router will talk to you, it is instantly replaced with a "good" DHCP local IP address in the range the router is dispensing. This happens so fast most users would swear there is no such thing as "self-assigned" IP address -- they have never seen one.

When you are stuck with self-assigned or none, it means no one will talk to you to give you a better address. On Ethernet, this is caused by really fundamental problems like broken cables.

On Wifi, you have failed to make the connection to the Main Router for some reason. This is not likely to be a simple mis-configuration on your Mac -- much more likely to be a fundamental error setting up the repeater portion of the Network.