Foreign keys work by joining a column to a unique key in another table, and that unique key must be defined as some form of unique index, be it the primary key, or some other unique index.
At the moment, the only unique index you have is a compound one on
ISBN, Title which is your primary key.
There are a number of options open to you, depending on exactly what BookTitle holds and the relationship of the data within it.
I would hazard a guess that the ISBN is unique for each row in BookTitle. ON the assumption this is the case, then change your primary key to be only on ISBN, and change BookCopy so that instead of Title you have ISBN and join on that.
If you need to keep your primary key as
ISBN, Title then you either need to store the ISBN in BookCopy as well as the Title, and foreign key on both columns, OR you need to create a unique index on BookTitle(Title) as a distinct index.
More generally, you need to make sure that the column or columns you have in your
REFERENCES clause match exactly a unique index in the parent table: in your case it fails because you do not have a single unique index on