The “See size chart” element is not a button, it is actually plain text with a button-like style. You need to change it’s CSS rules

If you want it to be simple black text on white background, change this:

.yith-wcpsc-product-size-chart-button{
    background: #0f0f0f;
    border: none;
    border-radius: 0px;
    color: #ffffff;
    display: inline-block;
    padding: 3px 10px 3px 10px;
    text-decoration: none;
    margin: 5px 3px;
    cursor: pointer;
    box-shadow: 0px 2px 7px -2px #dddddd;
}

to

.yith-wcpsc-product-size-chart-button {
    color: #000000;
    display: inline-block;
    cursor: pointer;
}

and remove this

.yith-wcpsc-product-size-chart-button:hover {
    background: #3f3f3f;
}

I’ve got a button. And this button when clicked, will produce popup. The problem is I don’t want it to be a button, instead, I just want it to be a plain text. All the function will be the same.

Is there any way to get around this?