Generally, you'll need to register a company, and before you can operate a tiur related agency, your company need to get a licence from the Singapore Tourism Board.
When it comes to the denial reason, is this always included in the rejection form? My employer says it only said "find a more suitable candidate" or something along those lines. @Wusthof It is difficult to know specific reason unless your potential employer is willing to show you the rejection lett...
Your potential employer should be able to address the rejection reason(s) before attempting an appeal. Generally, if initial self-assessment checklist qualifies, the application should "normally" be approved.