Does it work if you switch the data source from a query to a table?

What data type is Category?

Good catch, Alex. I didn't even think about that because I'm so used to Combo Boxes always having IDs in them, but if your CategoryCombo has TEXT as the bound column, remember you need to enclose it in quotes.

Category is Short Text.
So, I should set the bound column on the Combo to CategoryID, then.
As a matter of course I think that's best.
I'll do both and let you know.

Thank you both, very much.

Yep. IDs are best. If you do use text, you need to say:

SQL = "WHERE Category = '" & CategoryCombo & "'"

Unless your Category might have an ' in it, in which case:

SQL = "WHERE Category = """ & CategoryCombo & """"

Quotes can be tricky. I didn't used to mind too much about whether to do the ' or the "" until I started working with SQL Server which is much more picky. And I started dating a girl named D'Angelo and realized my Customer database might have a problem with her name in places. LOL

So why is it then, that MsgBox SQL returns a number, ie Category = 3.

You could be storing a number in a text field. What are the columns for your combo box? Let me see the RowSource property.

1- Changed to IdCategory and it worked like a charm.
2- Tried the quotes, and not so much. I'm just not fluent in quotes.

3- Row source, this is the Sub that I just changed to, an worked.

If IsNull(CategoryCombo) Then
        WhereStr = " "
    Else
        WhereStr = "Where IdCategory = " & CategoryCombo & " "

    End If
    
    NivelLista.RowSource = "SELECT IdProduct, IdCategory, Date, Category, Level, Class " & _
                           "FROM ProgressQ " & _
                           WhereStr & _
                           "ORDER BY Category, Level, Class;"

Before I had, Category instead of IdCategory, in WhereStr on the Else part.
On RowSource, IdCategory wasn't there.
When IdCategory was added, also changed tweaked the property sheet of the ListBox to account for the new column.

Yeah your best bet is to use the ID

It's good to know that the culprit was the field type.

Thank you Alex and Richard.
BTW This comes up in SQL 2 Class3, were Category data type is number.
As usual, my bad.

Hey - don't worry about it. That's what you're here for: to learn! You will remember this now even more because you made that mistake. That's what I'm here for: to help you learn. Never be embarrassed or afraid to ask a question.

Ooooohhhh I see plenty of embarrassment to come in my Access near future, Seor Richard.

My confidence is way up, now that I know that my lap top is fine and Access is installed properly.
After a while those two start creeping up the thought process and keep me away form paying attention the real culprits,
Quotes and data types.