have wifi but cannot connect to the internet on an iMac 24"

Sorry, yes that is what you stated in your original post. Hmm, maybe not enough coffee on my end this morning.

Anyways, have you tried temporarily connecting your iMac to your local router via an Ethernet cable?

I am, of course assuming that your iMac is currently connected by Wi-Fi. If not, before you do try Ethernet, please provide the values you find when holding down the option key, and then, clicking on the Wi-Fi icon: RSSI, Noise, Tx Rate, PHY Mode, MCS Index, & NSS.

What is the make & model of your wireless router? Per chance do you have it configured to use MAC address blocking?

Run thru the steps at > If your Mac isn't connecting to the internet over Wi-Fi - Apple Support

It that doesn't help, go to Apple > System Setting > Network > Firewall and turn off the Firewall.

If that fixes it and you feel the need to turn the Firewall back on?

Select Options... under Firewall and look for the Blocked Wi-Fi connection.

Did you make a new “Clean Test” account, restart w that account?

It could mean the iMac hasn’t gotten an IP address from the router. Could you snap the WiFi details showing the IP address?

Not good enough, unfortunately. With the Option key pressed, click on the Wifi icon on the top right, snapshot it and post here please. It doesn't reveal anything personal, don't worry, the IP address is a private one. And while you're at it, could you also specify make and model of the router? Thank you.

Have you installed and run any "cleaning", "optimizing", "speed-up", anti-virus or VPN apps on your iMac?

It may sound quite obvious, but it still gets me sometimes. I have the same problem on my MacBook Air but I just have to restart it sometimes or in some harder cases I've backed up the drive and I've signed in and out of everything.

Did you BACK-up and reinstall your OS?

You state that you can connect your devices to your local Wi-Fi network, but none of them can gain Internet access ... correct?

Let's start with a simple test using your MacBook Air.

Open your favorite web browser, and then, enter cloudflare.com in the browser's address bar. What is the result? If the Cloudflare home page doesn't appear, then repeat this, but this time, enter 1.1.1.1 in the address bar. Does the Cloudflare page appear now? If it does, then the issue is that your network clients are not getting the proper DNS settings to access the Internet. Think of DNS like the old "yellow pages" for the Internet.