iMac not detecting android phone when connected

1. Connect your Android to your Mac via USB.
2. Unlock your Android's screen. ... 
3. Swipe down to open the Android Notification Panel.
4. Tap the USB option in the Notification Panel.
5. Tap "File transfer" or "MTP."
6. Click the Go menu and select "Applications."
7. Double-click "Android File Transfer."

https://www.wikihow.tech/Connect-Android-to-a-Mac