Hi There,
Sorry for the confusion, so it was supposed to open a lightbox? Actually, it is opening a lightbox but nothing shows up because your post has no featured image (https://prnt.sc/hqtt76).
The other thing is, you need to click on the space around the “read more” it’s seems something is preventing the “read more” itself from being clicked.
Please do a testing for a plugin conflict. You can do this by deactivating all third-party plugins, and see if the problem remains. If it’s fixed, you’ll know a plugin caused the problem, and you can narrow down which one by reactivating them one at a time.
Let us know how it goes!
Thanks,