Replace with this code:

const addLinks = (searchPhrase, hyperlink) => {
    if (!searchPhrase) {
        return;
    }

    const document = DocumentApp.getActiveDocument();
    const body = document.getBody();
    let search = null;
    let count = 0;

    while (count < 1) {
        search = body.findText(searchPhrase, search ? search : null);
        if (!search) {
            break;
        }
        const searchElement = search.getElement();
        const startIndex = search.getStartOffset();
        const endIndex = search.getEndOffsetInclusive();
        searchElement.asText().setLinkUrl(startIndex, endIndex, hyperlink);
        count++;
    }

    document.saveAndClose();
};