You can't create a relationship between these two columns because one of the columns must have unique values.

A one-to-many relationship needs a unique key on the one-side, and the column you picked has duplicates.

Why it happens

• The dimension's key column contains duplicates.
• Both columns have duplicates, so the intended relationship is really many-to-many.
• Duplicates introduced by unnormalised keys — the same page split by query strings, casing and trailing slashes.

How to fix it

1. Deduplicate the dimension to a genuinely unique key (Group By, or Remove Duplicates after normalising).
2. Normalise the key first: strip query strings, lowercase, and unify trailing slashes so variants collapse.
3. For a genuine many-to-many, introduce a bridge table rather than forcing the relationship.