I have tried so many variations, both before and after I saw this thread: Using Basic Logic in Checkbox Formulas (https://developer.salesforce.com/forums/?id=906F0000000MIOYIA4)

After testing it out myself I can:

• Create an opportunity without a close date and stage is not closed
• Create an opportunity with a close date and stage is not closed
  • Whenever
• Create an opportunity with a close date and stage is closed
  • Only in the future

This is the formula I have settled on:

AND(NOT(IsClosed), CloseDate < TODAY())

Please, someone tell me I am doing this wrong, and then how I should do it correctly, or that someone can fix this Hands-On Challenge: https://trailhead.salesforce.com/en/content/learn/modules/advanced_formulas/checkbox_logic_formulas

I have tried so many variations, both before and after I saw this thread: Using Basic Logic in Checkbox Formulas (https://developer.salesforce.com/forums/?id=906F0000000MIOYIA4)

After testing it out myself I can:

• Create an opportunity without a close date and stage is not closed
• Create an opportunity with a close date and stage is not closed
  • Whenever
• Create an opportunity with a close date and stage is closed
  • Only in the future

This is the formula I have settled on:

AND(NOT(IsClosed), CloseDate < TODAY())

Please, someone tell me I am doing this wrong, and then how I should do it correctly, or that someone can fix this Hands-On Challenge: https://trailhead.salesforce.com/en/content/learn/modules/advanced_formulas/checkbox_logic_formulas

Admin Advanced >  Advanced Formulas > Using Basic Logic in Checkbox Formulas

The formula I used is:

NOT(IsClosed) && (CloseDate <= (Today() - 1))

Also tried:

AND(NOT(IsClosed),(CloseDate <= (Today() - 1)))

The error code I get is:
When I test it out myself, it's working as expected. Any ideas on what it could be?