Option Compare Database
Private Sub ApplyCombinedFilter(ByVal NewCriteria As String)

    Dim CurrentFilter As String
    Dim CombinedFilter As String
    
    
    ' Get the current filter string (if any)
    CurrentFilter = Me.Filter
    
    ' Check if a filter is already applied
    If CurrentFilter <> "" Then
        ' Combine the existing filter with the new criteria using "AND"
        ' Ensure a space is present before appending
        CombinedFilter = CurrentFilter & " AND " & NewCriteria
    Else
        ' If no filter exists, the new criteria is the entire filter
        CombinedFilter = NewCriteria
    End If
    
    ' Apply the combined filter to the form
    Me.Filter = CombinedFilter
    Me.FilterOn = True
    
End Sub

Private Sub FilterActive_Button_Click()

    ' Ensure the new criteria is a valid SQL WHERE clause fragment
    ' String values need single quotes: ' '
    
    Call ApplyCombinedFilter("[IsActive] = '1'")

End Sub

The error is happening on the line:

Me.Filter = CombinedFilter

Any help would be greatly appreciated.

What is the error?

Run-Time Error "3709":
The search key was not found in any record.

IsActive is a Boolean value and should not be enclosed in quotes.

Call ApplyCombinedFilter("[IsActive] = True")

or

Call ApplyCombinedFilter("[IsActive] = 1")

By the wa; the field that was called "Active" and now has been renamed "IsActive", is a Combo Box.  I have follow Richard class instructions to create a separate Table that stores the different employee’s status; Active, Terminated, Inactive (Disability), and Transferred. The Table is called UserStatus-Table.
The ComboBox gets it's data from that Table.

If it is a ComboBox you will need to access the Column property.

IsActive.Column(1)

Now I'm getting:

Compile Error.
Syntax Error

On this line:

Private Sub FilterActive_Button_Click()

And the last line is in Red:

Call ApplyCombinedFilter IsActive.Column(1)

ApplyCombinedFilter "IsActive.Column(1) = 1"

Back to this error message:

Run-Time Error "3709":
The search key was not found in any record.

And this line highlighted:

Me.Filter = CombinedFilter

Amir You should be looking for the text of the combo box, not a True/False indication.  In your example, only "Active" status would be the word "Active" any of the other status values would be an inactive status.

ApplyCombinedFilter "IsActive.Column(1) = " & "Terminated"