Changing a column data type results in an index that is too large
Solution 1:
The 900-byte limit on the indexed field size is inherent to SQL server, so you can't change that.
NVARCHAR() columns take two bytes per character, so you could resize your column to 450 instead of 900.
You could change the data type from NVARCHAR to VARCHAR, as long as your data is in Western European languages. That is probably not a good assumption.