That error is being triggered because the author ID that you're trying to reassign the entry to is not one of the designated valid authors for the channel. To fix this you'll need to edit the member group (of the user you're trying to assign the entry to) and enable the 'Include Members in PUBLISH page multi-author list?' setting.
You may also need to enable that setting for each individual user via Member Administration > Edit Member > Member Preferences > "Include user in PUBLISH page multi-author list?"