通过查看Wicked PDF文档中的示例，似乎您在render_to_string方法中缺少了layout选项。
我建议这样做：

if params[:invoice_id].presence
  @invoice = Invoice.find(params[:invoice_id])
  attachments['invoice.pdf'] = WickedPdf.new.pdf_from_string(
    render_to_string(
      template: 'invoices/service',
      locals: { current_user: },
      # add this line and change the value with the right pdf layout path
      layout: 'layouts/pdf' 
    )
  )
end