So far this is what I have come up with
AND(ispickval('Status' NOT, "none"), Len(Text(App))=0 ))
I'm trying to create a validation rule that will not allow a user to create a new case if the 'App' doesn't have an option selected.  The 'status' field is a pick list as well as the 'app' field.  When a case is created the status by default is  new
 

I'm trying to complete the Hands-on Challenge for Manage Your Picklist Values, and I'm getting the following message "... Couldn’t find picklist formula with the correct information. Please double check the instructions" I used the formula provided:

Use the formula editor to set the Default Value for the Macaron Flavor picklist as follows:CASE(MONTH(TODAY()),1, "Gingerbread",2, "Strawberry",4, "Chocolate",7, "Raspberry",11, "Pumpkin",12, "Mint", "Vanilla")

These were the only instructions, 

As an administrator, you want the default value for the macaron flavor to be the flavor of the month for some months, otherwise, the default is vanilla.

I'm not quite sure what I'm doing wrong.

I received this message "There was an unexpected error while verifying this challenge. Usually, this is due to some pre-existing configuration or code in the challenge Org. We recommend using a new Developer Edition (DE) to check this challenge. If you're using a new DE and seeing this error, please post to the developer forums and reference error id: SAAHTAIR"  I tried logging in and out and waiting a few hours I completed the task but it's not able to complete in the trailhead.

So far this is what I have come up with
AND(ispickval('Status' NOT, "none"), Len(Text(App))=0 ))
I'm trying to create a validation rule that will not allow a user to create a new case if the 'App' doesn't have an option selected.  The 'status' field is a pick list as well as the 'app' field.  When a case is created the status by default is  new
 

I'm trying to complete the Hands-on Challenge for Manage Your Picklist Values, and I'm getting the following message "... Couldn’t find picklist formula with the correct information. Please double check the instructions" I used the formula provided:

Use the formula editor to set the Default Value for the Macaron Flavor picklist as follows:CASE(MONTH(TODAY()),1, "Gingerbread",2, "Strawberry",4, "Chocolate",7, "Raspberry",11, "Pumpkin",12, "Mint", "Vanilla")

These were the only instructions, 

As an administrator, you want the default value for the macaron flavor to be the flavor of the month for some months, otherwise, the default is vanilla.

I'm not quite sure what I'm doing wrong.