This error message indicates that the VAT code of one or more booking lines in combination with the used general ledger accounts is not valid in SnelStart. This is related to VAT function restrictions in SnelStart.
Download the list of allowed VAT codes here.
Example: for a general ledger account with function 32 (purchases/costs high (excluding VAT; section 5b)), only VAT code "Purchases high (8)" may be used in Zenvoices. When another VAT code is used, the previously mentioned error message will appear.
More explanation about functions of general ledger accounts in SnelStart can be found in the following article: https://kennisplein.snelstart.nl/klanten/s/article/Instellingen-Grootboekrekeningen-Functie-van-een-grootboekrekening.