Sharepoint library metadata adding a calculated field based on another lookup field

MarM79 Not sure what are you trying to do with JSON, but you can use lookup columns in JSON without any problem. JSON formatting works with internal name of the columns. So, make sure you are using correct internal names for your columns. Follow this article to get internal name of your SharePoint column: Find the Internal name of columns in SharePoint Online 

Also, you would like to use the lookValue instead of lookupId to get the actual value like: 

{
    "$schema": "https://developer.microsoft.com/json-schemas/sp/v2/column-formatting.schema.json",
    "elmType": "div",
    "txtContent": "=[$SupplierNumber.lookupValue] + '-' + [$InvoiceNumber]"
}

Documentation: Use column formatting in SharePoint  

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