I am unable to open or create new boards in Freeform on iMac

Give this a try: boot into Safe Mode according to How to use safe mode on your Mac and test to see if the problem persists. Reboot normally and test again.

NOTE 1: Safe Mode boot can take up to 3 - 5 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

NOTE 2: if you have a wireless keyboard with rechargeable batteries connect it with its charging cable before booting into Safe Mode. This makes it act as a wired keyboard as will insure a successful boot into Safe Mode.

If the Hard Disk is indicating SMART errors (confirm this using DriveDX) that means the HD is failing. First thing to do is BACKUP using Time Machine if you are not already. If you already use Time Machine then test it (restore any file) to ensure it is operating okay.

Next is if you want to keep the Mac you need to get the failing HD out of the equation. The easiest and most cost effective method is to use an SSD as your startup drive. Another poster has provided excellent instructions for doing so, these are in How to setup an external SSD as your startup disk. The SSD I. would recommend contacting OWC at www.macsales.com to help them select an external SSD for your needs.