Answers for "unique constraint on foreign key"

there is no unique constraint matching given keys for referenced table

It's because the name column on the bar table does not have the UNIQUE constraint.

So imagine you have 2 rows on the bar table that contain the name 'ams' and you insert a row on baz with 'ams' on bar_fk, which row on bar would it be referring since there are two rows matching?

unique key

Unique Key:
Only unique value and also can contain NULL