Error: clickable element "ADD" was not found by text|CSS|xpath

I have created a website using html css php and some javascript along with a database and it works perfectly fine all the buttons and elements are present there're no errors and it functions as it should. the problem is that i have been asked to test it with an AutoQA tester and even though it functions as it should i got this error from the tester and i don't know how to fix it

<pre>
Error: Clickable element "ADD" was not found by text|CSS|XPath
    at new ElementNotFound (node_modules/codeceptjs/lib/helper/errors/ElementNotFound.js:14:11)
    at assertElementExists (node_modules/codeceptjs/lib/helper/WebDriver.js:2770:11)
    at WebDriver.click (node_modules/codeceptjs/lib/helper/WebDriver.js:859:7)
    at processTicksAndRejections (internal/process/task_queues.js:97:5)
I }) => {
        I.amOnPage('/');
        I.click('ADD');
        I.seeElement('#product_form');
        I.fillField('#sku', 'SKUTest000');
        I.fillField('#name', 'NameTest000');
        I.fillField('#price', '25');
        I.selectOption('#productType','DVD');
        I.fillField('#size','200');
        I.click('Save');