Hi @areej, based on your screenshot it looks like you are having two items here. One with a binary property called data1 and another item with a binary property data2.
So when your Send Email node runs for the first item, it would fail to find the data2 property you are referencing in the Attachments field. On the second item it would fail to find data1 instead.
So before sending out your email, you would want to merge this two separate items into a single item. On your screenshot it seems you are already using a Merge node. Is there a chance you are simply using your Merge node in “Append” mode? If so, can you try switching it to “Combine” mode and pick “Merge By Position” like so? This should return a single item combining the properties from your incoming items rather than two separate items: