Items with trading partner validation errors are noted by a red exclamation mark. There are a couple of different ways you can see why your items are invalid.
On the 'Items' tab you can click on the red exclamation mark to see the errors for that specific item.
You can click on the 'Errors' tab to see a list of all items that have errors, as well as the number of errors, and number of trading partners impacted. To see the error text you can click on the item.
Tip: Most of the time items have the same errors as other items. It's easiest to address one type of error for all items in your spreadsheet then move on to the next error.
Once you have identified what is in error you update your spreadsheet with the correct information or send a corrected integrated document. Only items in a status of Valid will be transmitted to your trading partner.