Beep every 5 seconds when I turn on my iMac

For what it's worth, a beep every 5 seconds indicates a Mac isn't detecting any RAM. First, make sure it's properly installed. If it is, it's either defective or the wrong RAM. You can't "fix" it, it would need replaced.

what model year of iMac do you have?

have you replaced RAM, or add additional memory sticks?

if yes, what vendor supplied the new RAM? and what are the specs of them?

Read If your Mac beeps during startup - Apple Support

John Galt wrote:

Read If your Mac beeps during startup - Apple Support

I did. 😎