Hi, can someone please advise what is the correct syntax for a Flow entry condition formula to check if a picklist value has NOT been selected?

I have seen many posts saying that the following formula should work, although this only appears to work for a field Validation Rule formula:

ISBLANK(TEXT(picklist_field))

When I attempt to use this syntax, I get the following error message when attempting to debug the flow:

Unfortunately, there was a problem. Please try again. If the problem continues, get in touch with your administrator with the error ID shown here and any other related details. Error ID: 1972618629-108722 (2126388401)

I am having trouble completing the following Trailhead: Create a formula field that determines the number of days between the end date of a contract and today. This is the formula that I am using:

EndDate - TODAY() 

I keep getting the following error when I check the syntax:  Error: Formula result is data type (Number), incompatible with expected data type (Date).