This is because you have explained a transaction in FreeAgent with a category that is not available as part of the standard category list in the Mettle app.
This usually happens when;
you have added an additional custom category in FreeAgent
you have edited the category name or number in FreeAgent
it is a FreeAgent category which Mettle doesn’t support yet
If you want to view the category assigned to the transaction you should log into your FreeAgent account.