Cant get substrategy box when creating valuation variant

I am trying to create a valuation variant and want to use strategy L (Price from purchase info record) as the 1st strategy in material valuation.

As soon as I enter it I get error KQ024 - Enter substrategy for purchasing info record valuation.

I cannot get past this error....no substrategy box appears on the screen for me to complete, and there appears to be no menu options for adding in the substrategy boxes.

There are no standard valuation variants with a substrategy to copy to get round the error.

How do I get past the error and get a substrategy box to appear so I can complete it?

Thanks

Graham