How send text to 5 digit number

You can't.

Probably the number you are trying to send an SMS is a one-way SMS. You may receive but cannot reply.

Types of SMS services known to me.

One-way SMS: A business may have something to say, so they may send a single text message blast to get the word out. For example; Promotional SMS, Notices, Verification Codes, Bank OTPs, Transaction Confirmation, offers, Coupon, Discounts, Flash SMS, Reminders etc

Two-way SMS: This is what most of us use.

SMS Chatbots: The chatbot will allow an organisation's clients to start or stop a service or interact with FAQs.