What this error message mean and How to solve it ?

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SQL

server

Hi, i create these three tables one is
department(dep_no int PK , dep_name nvarchar ), -- parent table of LEVEL
second table is LEVEL(lvl_dep_key int PK , dep_no int FK & PK , lvl _id nvarchar Pk) -- parent table of courses & the child table of department
i make the lvl_dep_key as a super key for both lvl_id and dep_no columns
to relate it to the children tables instead of taken both lvl_id and dep_no columns to these tables
third table is courses( lvl_dep_key int FK & PK , cors_id nvarchar PK, cors_name nvarchar,price money) -- the child table of LEVEL

so when i tried to create the relationship between LEVEL table and courses table by these columns which is (lvl_dep_key of LEVEL table , and lvl_dep_key of courses table i get this error message " the columns in table [LEVEL] do not match an existing primary key or UNIQUE constraint ", i googled to know what it's mean but no benefit . so can any help me to solve this problem plzzzzzzzzzz. thanks