JSFiddle - React, Tailwind, and code Playground

HTML

<select class="SomeOtherBox">
    <option>--</option>
    <option>1</option>
    <option>2</option>
</select>

<input type="text" class="NumbersOnly" />

CSS

.TextBoxError{
    background:red;
    color:white;
}

JavaScript

$(Start);

function Start() {
 
    $('body').on({
        keyup: function () { TestNumbersOnly($(this));},
        focus: function () { SomeFunction($(this)); }
    }, '.NumbersOnly');
}

function TestNumbersOnly(TheBox) {

    var TheInput = TheBox.val();
	var TheCleanInput = TheInput.replace(/\D/g, '');

	TheBox.val(TheCleanInput);
}

function SomeFunction(TheBox) {
    
    	var TheOtherBox = TheBox.prev('.SomeOtherBox');

	if ($.isNumeric(TheOtherBox.val()) === false) {

		TheBox.blur();

		TheOtherBox.addClass('TextBoxError');

		setTimeout(function () {

			TheOtherBox.removeClass('TextBoxError');

		}, 500);
	}
}