You can filter by last invoice number, but this is not recommended as there can be a one-to-many relationship between an order and invoices.
The approach I'd go with is to query invoice detail lines by Order (OE0400) - also, I'd suggest ensuring your order for the Browse is set to 3 - Order/Ord Dtl/Dtl

Thank you for the tips. So I ended up using OE0420 to get filter for Invoice Number and using FilterCount instead of Browse function

Function ValidateInvoiceNumber(invoiceNo As String) As Boolean
    
    Dim uniq As Boolean
    
    Dim mDBLinkCmpRW As AccpacCOMAPI.AccpacDBLink
    Set mDBLinkCmpRW = OpenDBLink(DBLINK_COMPANY, DBLINK_FLG_READWRITE)

    Dim mDBLinkSysRW As AccpacCOMAPI.AccpacDBLink
    Set mDBLinkSysRW = OpenDBLink(DBLINK_SYSTEM, DBLINK_FLG_READWRITE)
    
    Dim temp As Boolean
    Dim INVheader As AccpacCOMAPI.AccpacView
    Dim INVheaderFields As AccpacCOMAPI.AccpacViewFields
    mDBLinkCmpRW.OpenView "OE0420", INVheader
    Set INVheaderFields = INVheader.Fields
    
    If INVheader.FilterCount("INVNUMBER=" & invoiceNo, 0) > 0 Then
        Logger.LogDetails "Invoice Number=" & invoiceNo & " already exist."
        ValidateInvoiceNumber = False
    Else
        ValidateInvoiceNumber = True
    End If
End Function

Thank you for the tips. So I ended up using OE0420 to get filter for Invoice Number and using FilterCount instead of Browse function

Function ValidateInvoiceNumber(invoiceNo As String) As Boolean
    
    Dim uniq As Boolean
    
    Dim mDBLinkCmpRW As AccpacCOMAPI.AccpacDBLink
    Set mDBLinkCmpRW = OpenDBLink(DBLINK_COMPANY, DBLINK_FLG_READWRITE)

    Dim mDBLinkSysRW As AccpacCOMAPI.AccpacDBLink
    Set mDBLinkSysRW = OpenDBLink(DBLINK_SYSTEM, DBLINK_FLG_READWRITE)
    
    Dim temp As Boolean
    Dim INVheader As AccpacCOMAPI.AccpacView
    Dim INVheaderFields As AccpacCOMAPI.AccpacViewFields
    mDBLinkCmpRW.OpenView "OE0420", INVheader
    Set INVheaderFields = INVheader.Fields
    
    If INVheader.FilterCount("INVNUMBER=" & invoiceNo, 0) > 0 Then
        Logger.LogDetails "Invoice Number=" & invoiceNo & " already exist."
        ValidateInvoiceNumber = False
    Else
        ValidateInvoiceNumber = True
    End If
End Function