Check in BC that the mapping between your setup BC and PEPPOL codes matches the list of units of measure on the below link

This is how it's done from  Danish GREEN to PEPPOL RED

The official PEPPOL unit of measure list can be found below

Peppol BIS Billing 3.0 | Rules

The error will occur if the setup is incorrect

{"message":"Invalid Document","errors":[{"context":"/Invoice[1]/InvoiceLine[1]/InvoicedQuantity[1]","pattern":"","text":"[BR-CL-23] [BR-CL-23]-Unit code MUST be coded according to the UN/ECE Recommendation 20 with\n      Rec 21 extension","isWarning":false},{"context":"/Invoice[1]/InvoiceLine[1]/Price[1]/BaseQuantity[1]","pattern":"","text":"[BR-CL-23] [BR-CL-23]-Unit code MUST be coded according to the UN/ECE Recommendation 20 with\n      Rec 21 extension","isWarning":false}]}'