Hi,
This error is raised because the tax at header and item level should not be different.
In order to raise this error in SRM side, please activate the following customizing:
On IMG (trx SPRO), follow the path:
> Supplier Relationship Management
> SRM Server
> Cross-Application Basic Settings
> Message Control
> Influence Message Control for Invoice Simulation
This error can't be avoided if you are using different tax codes in header and item levels.
Regards,
Ivy