error:InvalidValueError: setCenter: not a LatLng or LatLngLiteral: in property lat: not a number

The .value attribute of a HTMLInputElement returns the value as a string.

You have to parse the content of lat and lng with parseFloat() before passing it to the maps API

function initAutocomplete() {
    var lat = parseFloat(document.getElementById('lat').value);
    var lng = parseFloat(document.getElementById('lng').value);

    var map = new google.maps.Map(document.getElementById('map'), {
        center: {
            lat: lat,
            lng: lng
        },
        zoom: 13,
        mapTypeId: 'roadmap'
    });
}