This error occurs because there's a zero value for a performer on the contract but no authorised leave (i.e. Maternity, Paternity, Sick Leave) present. 
The Provider will need to enter a value for the period if the performer isn't on authorised leave as each performer needs to have either Net Pensionable Earnings (NPE) or Net Pensionable Earnings Equivalent (NPEE) recorded against them.
There's a box in the NPE / NPEE field called 'Authorised Leave' if you click on this it shows whether the performer is on authorised leave or not.