Is there a way around an Invalid Operation when using Dates generated by formulas

Hi,

I am trying to use this formula =IF([Next inspection due]@row <= TODAY(0), 1, 0) then mark it with a flag. in the Flag Due Date column.

Every time I use this formula I get INVALID OPERATION because the Next inspection due column is created from a formula of a formula of a date.

Date6 is add on 6 months, Date1 is add on 1 year, and Date2 is add on 2 years. Next inspection date then checks if they got an A, B, or a C. This will then determine when their next inspection is due.

Formulas used:

Date6 - =IFERROR(DATE(YEAR([Date of last Inspection]@row), MONTH([Date of last Inspection]@row) + 6, DAY([Date of last Inspection]@row)), DATE(YEAR([Date of last Inspection]@row) + 1, MONTH([Date of last Inspection]@row) - 6, DAY([Date of last Inspection]@row)))

Date1 - =IFERROR(DATE(YEAR([Date of last Inspection]@row) + 1, MONTH([Date of last Inspection]@row), DAY([Date of last Inspection]@row)), DATE(YEAR([Date of last Inspection]@row), MONTH([Date of last Inspection]@row), DAY([Date of last Inspection]@row)))

Date2 - =IFERROR(DATE(YEAR([Date of last Inspection]@row) + 2, MONTH([Date of last Inspection]@row), DAY([Date of last Inspection]@row)), DATE(YEAR([Date of last Inspection]@row), MONTH([Date of last Inspection]@row), DAY([Date of last Inspection]@row)))

Next inspection due - =IF(HAS(Grade@row, "A"), JOIN([Date6]@row), IF(HAS(Grade@row, "B"), JOIN([Date1]@row), IF(HAS(Grade@row, "C"), JOIN([Date2]@row), "")))

Flag Due Date - =IF([Next inspection due]@row <= TODAY(0), 1, 0)

This last one gives me the Invalid Operation.

I have tried JOIN but that results in the same problem. I have tried on another sheet just replicating the last step with a JOIN and it will work then. It seems to be with all of the formulas together it will not work.

Thank you for reading this and I would greatly appreciate any help.