Cannot sent text messages

I have just bought a Redmi 5 phone, running Android 7.1.2. when sending some messages I am getting the error "failed" "messages couldn't be sent". Can anyone please help me solve this problem? I think it may be due to SMS premium not being set but I cannot find the Messaging Service app. I have looked for a solution on the 'net but none can fix my problem. Please help. TIA

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