[рони]

voraa,
ready внутри if ... странная конструкция.
возможно так ...

document.addEventListener("DOMContentLoaded", function() {
            let select = document.querySelector('#select_vid_doc1');
            let element = document.getElementById("seria1");
            let maskOptions = {
                mask: "000000000000"
            };
            let mask;
            if (select && element) {
                select.addEventListener('change', function() {
                    let val = this.value;
                    if (val == 1) mask = new IMask(element, maskOptions);
                    else if (mask) mask.destroy();
                })
            }
        });