SAP Message 60826 - Asset & not defined with main no. '0000'

Diagnosis
You have entered an asset number which does not have a main number'0000'.

System Response
Your entry is not accepted.

Procedure
A Real Estate asset must be assigned a business entity. This RealEstate asset must have the sub-number '0000' which defines it as a mainnumber. Select another asset or define the Real Estate asset requiredin the asset accounting.