Hi @cchan000
First, you need to add this lookup filter based on a key other than Attachments, so this should refer to some other property, let's say title:
To resolve the red cross, use expression: LookUp('MyDataSource',Title = TitleControl.Text).Attachments
Here, you need to replace the highlighted part with the control name that is storing Title value.
Second, to add a label, click on the "Add an Item from the Insert Tab", make sure this select the first item of the gallery. Now, from the Insert tab at the top, insert a text label and you need to use the other expressions on that Text Label. Through this, you should be able to text label in each row of the gallery control.
Hope this Helps!
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