There are quite a few known issues with the table editor in SSMS. Most people will not utilize it because of the issues that can occur - and instead utilize insert/update/delete statements in SQL.
One of the most prevalent issues is that the GUI cannot update a table if that table does not have a primary key or unique key defined. Without being able to determine what column(s) uniquely identify the row - the GUI cannot determine how to update the row.
For example - you have the column FBA-ID - and you can see that these are different values, but the GUI does not know that...the SQL to update that row would be:
UPDATE dbo.[FBA Items]
SET [UPC / Core SKU] = '2222'
WHERE FBA-ID = 'rr';
But - since that column is not defined as a PK or UK then SQL Server does not know how to build the WHERE clause to update that row.
If you had 2 (or more) rows where FBA-ID is equal to 'rr' - which row in the GUI is being updated?