My wife and I both have 2017 iMacs and hers has developed an issue of going to a lock screen literally every minute or so at times and requiring a password.

Please restart the computer in Safe Mode per the directions in the  How to use safe mode on your Mac - Apple Support link. 

Note: If you are using a wireless keyboard please either plug it in using the supplied Lightning cable that shipped with it. If you are using a legacy keyboard or a third-party keyboard that does not have re-charging abilities, please use any USB keyboard. Also note that a Safe Mode restart may require you to hold down the shift key for up to 5 minutes before the computer starts in Safe Mode. You will know it is in Safe Mode when you see “Safe Mode” in the upper right of the display.

After restarting in Safe Mode, then restart normally and test. If it does not work, please let us know the troubleshooting you have attempted so far?

Not sure if these things will help, but it's worth a look.

Visit the Security & Privacy preference settings.

Visit and adjust the Energy Saver settings.

Visit and adjust or disable the Screen Saver if it is setup.