CAUSE 1:
One of your sudoers entries contains a RunAs alias that has not been defined.
E.g
myuser ALL=(MYRUNAS_ALIAS) ALL
CAUSE 2
The entry in the RunAs section is actually a user, however you have used Capital letters for the name.
Therefore sudo believes that this is an alias.
Note that this error will also occur when using normal sudo and the 'visudo -c' command