const onChangeValidate= (e) => {
   var phone = e.target.value;
   if( !(phone.match('[0-9]{10}')) ){
     setPhoneNo(false);
     setErrorMessage("Please enter 10 digit")
    }else{
    setPhoneNo(true)
    }
}
<p>{setPhoneNo===false?errorMessage:''}</p>

Set like these if there will be setphone no false then msg will be shown 

You seem to understand the basic construct of a function component using state but there are a couple of issues.

1. All event handlers will receive that event as an argument. So you can write <input onChange ={onChangeValidate} /> and <button onClick={validateFunc}>Submit</button>.

2. It maybe better to use test rather than match in this instance: /[0-9]{10}/.test(phone).

But I also think that you should allow the form to do the validation for you, rather than JS. This way you don't need the error state as the validation is all handled automatically.

Wrap your input and button in a <form> element with its own onSubmit handler, make the input required, and add a type="submit" to the button. You can use CSS to change the style of the input if it's not valid.

const { useState } = React;

function Example() {

  const [phoneNo, setPhoneNo] = useState('');

  // Update the phone number state when the
  // input changes
  function handleChange(e) {
    setPhoneNo(e.target.value);
  }

  // In this example prevent the form from
  // submitting so we can log the state, but you'll probably
  // want to do an AJAX call here to submit the form
  function handleSubmit(e) {
    e.preventDefault();
    console.log(phoneNo);
  }

  // Add your regex pattern to the input
  // and make it required. If you submit an invalid
  // number you'll get a neat little tooltip warning
  // you of the issue, and the form won't submit until
  // it has been fixed 
  return (
    <form onSubmit={handleSubmit}>
      <input
        pattern="[0-9]{10}"
        placeholder="Add a phone number"
        onChange={handleChange}
        required
      />
      <button type="submit">Submit</button>
    </form>
  );

}

ReactDOM.render(
  <Example />,
  document.getElementById('react')
);

input:invalid { border: 1px solid red; }

<script src="https://cdnjs.cloudflare.com/ajax/libs/react/17.0.2/umd/react.production.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/17.0.2/umd/react-dom.production.min.js"></script>
<div id="react"></div>

Try this!

const [phoneNo, setPhoneNo] = useState(false)
const [errorMessage, setErrorMessage] = useState("")

const validateFunc = () => {
  if(phoneNo) {
    // perform submittion
  }
}

const onChangeValidate= (e) => {
   var phone = e.target.value;
   if( !(phone.match('[0-9]{10}')) ){
     setPhoneNo(false);
     setErrorMessage("Please enter 10 digit");
    }else{
     setPhoneNo(true);
     setErrorMessage("");
    }
    
   }




----
<input onChange ={(e) => onChangeValidate(e)} />
<button type="button" onClick = {validateFunc()}>Submit</button>
<p>{errorMessage}</p>