DLookup
After Update
AfterUpdate & DLookup

Kevin has you pointed in the right direction. Put the lookup in the AfterUpdate event of your PartNumber combo box. That's the event that fires after the user actually selects a different part.

Something like this:

Description = DLookup("Description", "InventoryT", "PartNumber=" & PartNumber)

Of course, adjust InventoryT, Description, and PartNumber to match your actual table and field names. If PartNumber is a text field rather than a number, it needs quotes around the value:

Description = DLookup("Description", "InventoryT", "PartNumber=""" & PartNumber & """")

Also, make sure your combo box is bound to the PartNumber field in your transaction table. If Description is simply informational and always comes from InventoryT, you may not need to store it in the transaction record at all. But if you need to preserve the description as it was at the time of the transaction, then copying it into the transaction record makes sense.

I tried multiple DLookup statements and finally it worked with Me.Description = DLookup("Description", "dbo_StockroomInventoryT", "PartNumber='" & Me.PartNumber & "'"). I found if I did not include the Me part then it would give errors. Thanks for the help

That's weird, because you normally only need Me when you're referring to properties or methods of the form itself. A bound control on the form should generally work just fine as:

Description = DLookup("Description", "dbo_StockroomInventoryT", "PartNumber=""" & PartNumber & """")

The fact that Me.Description worked suggests Access may have had trouble resolving Description as the control name. Make sure both PartNumber and Description are actual controls on the form, and that their Name properties are PartNumber and Description. The control name matters here, not just the underlying field name.

If one or both fields aren't currently on the form, try adding them to the form and then test again. Access sometimes gets a little quirky about recognizing controls in the form's module, especially if controls have been renamed or copied around. Using Me.Description explicitly tells Access, "use the Description control on this form," so it can clear up that ambiguity.

In any case, you got it working, which is the important part.

I should give some background information that may or may not clarify what I am doing. My Transactions form is in my FE Access database and the tables were in a BE Access database. I have now built the same tables in SQL Server Express, shout out to Richard for the first SQL Server class. The Record Source for my Transaction form is a query on the Transactions table. The PartNumber field on the form is a Combo Box, Control Source is PartNumber result in the query and the Row Source is a Select Statement. The Description field is a Text Box with Control Source as Description result in the query. I have checked the names of the fields on the form and in the tables, they all match. Working with SQL Server and my Access FE is a challenge and I am working through these as they present themselves. Cant wait for SQL Server for Access User class 2. Thanks

When Me is required to avoid a run-time error, it's usually due to a name conflict with a local variable or an argument of a procedure.  I always use Me whether it's needed or not, so that it's clear to the reader that it's from a form.