In this case, Otto expects valid values for the base price unit and the base price quantity, both of which you can find in the catalogue. Usually, the error occurs because one of the values has not been configured or is missing from your variant.
Make sure that meaningful values are stored in the catalogue for your variants.

Example:
You sell items in pieces and millilitre quantities.
Base price quantity
You must then store the base price quantities 1 and 100 in the catalogue and link them to your variants.

Base price unit
Valid values must be present in the catalogue and on your variants for all units you use.