How delete a record if certain criteria exists?

currentdb.execute "delete tblScheduleRecords  where UniqueID = '" & Me!txtUniqueID & "'" 

This datasheetview form, is that a subform?

Is this form bound to table tblScheduleRecords?

Gustav,
Yes, the datasheetview form is a sub-form.  Yes, it is bound to tblScheduleRecords

As you already have the record in the form, just delete it from the RecordsetClone without an external call:

Dim Records     As DAO.Recordset

If Me.NewRecord = True Then
    Set Records = Me.RecordsetClone

    Records.FindFirst "[UniqueID] = '" & Me!txtUniqueID.Value & "'"
    If Not Records.NoMatch Then
        Records.Delete
    End If
    Records.Close    
End If

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The record would not yet be in the form because the user gets a warning with:

Private Sub Form_BeforeUpdate(Cancel As Integer)
On Error GoTo Err_Form_BeforeUpdate

    If Me.NewRecord = True Then
        If (Not IsNull(DLookup("[UniqueID]", "tblScheduleRecords", "[UniqueID] ='" & Me!txtUniqueID & "'"))) Then
            MsgBox "This client has already been scheduled for this Group, Schedule Date & Time."
            Cancel = True
        End If
    End If

Exit_Form_BeforeUpdate:
    Exit Sub

Err_Form_BeforeUpdate:
    MsgBox "Error Number: " & Err.Number & vbCrLf & "Error Description: " & Err.Description & vbCrLf & "Error Source: " & Err.Source
    Resume Exit_Form_BeforeUpdate

End Sub

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I just want the user to get the warning, and then clear the record entry in the subform.

Then all you need is to cancel the insert of the new record:

Private Sub Form_BeforeInsert(Cancel As Integer)

    Dim Records     As DAO.Recordset

    Set Records = Me.RecordsetClone

    Records.FindFirst "[UniqueID] = '" & Me!txtUniqueID.Value & "'"
    If Not Records.NoMatch Then
        MsgBox "This client has already been scheduled for this Group, Schedule Date & Time."
        Cancel = True
    End If
    Records.Close    

End Sub

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I put your latest code in the beforeinsert event of the sub-form and it allows for a duplicate entry.  ??