The Mysql-auto_increment field must be a key

At first I thought I had to set the primary key, because only the ID primary key is the self-increment, later found that as long as the key is OK. so I tried to set it to a single constraint, and the result was successful.
CREATE TABLE testtable (
ID varchar (not NULL),
Apptype varchar (+) not NULL,
UUID int auto_increment not NULL UNIQUE,
CONSTRAINT Pk_lis_view_app PRIMARY KEY (id,apptype)
); It is obviously not possible to set the UUID as the primary key, but the requirement is key. Perhaps the constraint is also considered a key, so I have succeeded in adding a unique constraint to the definition of the UUID.

The Mysql-auto_increment field must be a key