Foreign keys can't be added because they exist but they don't exist!
You cannot add the constraints to tblUserRoles because the data within this table already violates the constraints that you are trying to add.

a) Double check that all UserID values in tblUserRoles reference UserID values in tblUsers

b) Do the same as the above for tblRoles and RoleID.

c) As I imagine both of the above will return records that are not in the referenced table, you'll need to clean up your data before adding the Foreign Keys

d) Name you foreign keys something useful! If I came to work on your database, how am I supposed to know what Relationship26 means?!