The European regulations MiFID2 and EMIR (and UK onshored versions) require all market participants to have a Legal Entity Identifier (LEI) prior to entering into transactions in in-scope financial instruments. The background to this requirement is set out in the factsheet from industry bodies ISDA and GFMA here and on the Financial Conduct Authority's (FCA) website here.
If you do not already have an LEI, it is necessary for you to obtain one or to provide a representation that your transactions are excluded from these regulations. If you do not have an LEI, and you do not confirm to us that your transactions are excluded, then you will be unable to trade financial instruments with NatWest Markets (NWM).