I found this forum and this blog that discusses the same problem. It is because the Display Name for the user has a "," in it.
The response from Microsoft is:
this is a known issue that when using a Task/Project Task list in the
gantt view, if you use People Picker control and choose a user name
with a comma in the “assigned to” field, it will shows “The user does
not exist or is not unique” as is displayed in the picture you
provided. We have taken efforts on this. Hope this issue can be
resolved in the next release of the product. For a workaround, you may
consider modifying the user name without comma in it.
Obviously it was not resolved and now the datasheet view in SP 2013 is using the same control. Since removing commas from names just so SharePoint could figure out a name was not an option but I was able to use the users e-mail address as that is unique on our farm.
Some other workarounds that were suggested:
Use the user’s alias instead of DisplayName when entering it into the row. The Gantt chart will accept the value in this case,
however this precludes use of the People Picker.
Where applicable, use the “New Item” option in the ribbon