The value of the cover field first has to be turned into a file/files object before you can retrieve its URL:
(toFile() if the field contains a single file; toFiles() if multiple, in which case you may e.g. want to use->toFiles()->first() to return the first of many)
Edit: Two more things to observe, just mentioning for completeness
- Before working with a file object, always check that a file exists:
<?php if ($cover = $page->cover()->toFiles()->first()) : ?>
<img src="<?= $cover->url() ?>" alt="">
<?php endif ?>
- Omitting the
altattribute in yourimgtag will make screen reader programs read out the entire URL, so for accessibility reasons this should be either empty (if the image content is not relevant) or a good description of what is depicted in the image.