Here are screencaps of the dropdown

You are exiting the Sub before you Requery the form.

Is CallSign Text or a Number?

Relational Combo

Alphanumeric, stored as text.  An example would be my FCC call sign W5FJN

Assumming CallSign is in the second column of your Combo Box:

    WHERE CallSign=""" & CallSignCombo.Column(1) & """"

Double Double Quotes

It would be a good idea to switch to a proper relational Combo Box rather the Multi-Select.
You will run into more problems later.

Evil Access Stuff

Richard's Multivalued Fields video shows how to move the data from a multi-valued field into relational tables. Once you get those moved into proper relational tables, having a multi-select list box built in a form isn't bad.

Richard discusses multi-select listboxes in Access Developer 15

Dan I wash my hands of you. You are dead to me, sinner.

LOL

Honestly, like the other guys are saying, I would rather you take some time to learn how to do it the right way with a proper relational table than try to fix what you've got there. It'd be a waste of all of our time. And I say that lovingly, as your teacher.

It's not that I don't want to help you. It'd be like if someone was trying to build a house using duct tape. I'm like, "Dude, just learn how to properly use nails."

But since we're here, just so you understand what the code is doing, you're trying to take all of the selected values from the multivalued combo box, convert them into one text string, and then write that string back into the same station record. That gives you two different representations of the same information, neither of which is properly relational.

The write conflict is probably happening because the form still has the current StationT record dirty, while your UPDATE query is trying to change that same record behind the form's back. On top of that, UseageIDCombo.Text and CallSignCombo.Value aren't actually being inserted into the SQL string, your CallSign needs quotes because it's text, and the Me.Requery can never run because it comes after Exit Sub.

We could spend time patching each of those problems, but we'd only be helping you build a more elaborate duct-tape house. The proper solution is a StationUsageT junction table with one record for each station and capability. Then you can still use a multi-select list box on the form if you like that interface, but the data underneath it remains properly relational.

As usual you are correct sir.  I've purchased Access Developer 15 and will use it and everyone's comments to correct my sin.  That is what I get for trying to take a short cut instead of doing it right the first time.

Atta boy. We're here if you need more help.