My WiFi connection doesn't recognize my router password
My Imac doesn't seem to recognize my wifi password
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If you are able to log in with the same password using other devices then try restarting your iMac and then try again. On rare occasions when I have had a similar issue that has worked for me.
If restarting doesn't work then try creating a new user to see if the issue is tied to a specific user.
How to test an issue in another user account on your Mac - Apple Support
If you are eventually able to connect to WiFi but continue having issues then this article may help:
If your Mac doesn't connect to the Internet over Wi-Fi - Apple Support