How to put a criteria for the function in a query? - vba

Question

I have a query where one of the fields is a function I wrote to get the due dates for trainings.
The function works and when I run my query it gives the correct dates but when I try to put in a criteria in the query to for example only output the due dates that are <31 days it gives me the error:
Data type mismatch in criteria expression
This is in the field for the query:
Due Date: getDueDate([Workday_Excel_Update].[Course Number])
This is the function:
Function getDueDate(CourseNum As String)
Dim yearCounter As Integer
Dim yearStart As Date
Dim yearNow As Integer
Dim monthFreq As Integer
Dim daysDue As Integer
'check if course number is part of the tracked trainings
If Not IsNull(DLookup("CourseName", "CourseNumbers", "CourseNumber = '" & CourseNum & "'")) Then
'if it is part of tracked training calculate date
monthFreq = DLookup("[Months]", "CourseNumbers", "CourseNumber = '" & CourseNum & "'")
yearStart = DLookup("[Frequency]", "CourseNumbers", "CourseNumber = '" & CourseNum & "'")
yearCounter = Year(yearStart)
yearNow = Year(Date)
While yearCounter < yearNow
yearStart = DateAdd("m", monthFreq, yearStart)
yearCounter = yearCounter + (monthFreq / 12)
Wend
daysDue = CInt(DateDiff("d", Date, yearStart))
'Else
'daysDue = 0
End If
getDueDate = daysDue
End Function
Example data:
CourseNumbers (Table)
CourseNumber
CourseName
Frequency
Months
1234
1234-training
1900-12-31
12
Workday_Excel_Update (Linked Table)
User ID
Badge ID
First Name
Last Name
Email Address
Primary Supervisor
Completion Date
Course Title
Course Number
Lesson Completion Status
0022334455
54321
Frank
Smith
frank.smith#work.com
John Leblanc
2021-05-20 2:59:54 PM
11234-training
1234
Not Started
SQL code of the Query:
SELECT Workday_Excel_Update.[User ID], Workday_Excel_Update.[Badge ID], Workday_Excel_Update.[First Name], Workday_Excel_Update.[Last Name], Workday_Excel_Update.[Email Address], Workday_Excel_Update.[Primary Supervisor], Workday_Excel_Update.[Completion Date], (getDueDate([Workday_Excel_Update].[Course Number])) AS [Days Until Due Date], Workday_Excel_Update.[Course Title], Workday_Excel_Update.[Lesson Completion Status]
FROM CourseNumbers INNER JOIN Workday_Excel_Update ON CourseNumbers.CourseNumber = Workday_Excel_Update.[Course Number]
WHERE (((Workday_Excel_Update.[Lesson Completion Status])<>"Completed" Or (Workday_Excel_Update.[Lesson Completion Status]) Is Null))
ORDER BY (getDueDate([Workday_Excel_Update].[Course Number]));
The getDueDate() function in the query will take as input a course number from another table which has a registration date for each employee. The registration date goes through a DateDiff() with the calculated due date and outputs an integer indicating the number of days left until the training is due or the number of days past the due date.
Query criteria
I also get the following error when I try to filter the query