Most likely this issue is caused by using Horizon WP theme with the latest WP version which is incompatible. Before removing the theme from sales we have posted this important notification:
We would like to inform you that we have discontinued updating this theme. Therefore please be advised that this theme will not work with the latest WordPress updates and we would like to highlight that its strongly recommended to use the theme with the appropriate WordPress version which is mentioned in theme details. Some of you would be disappointed about it, but you need to understand that this theme is outdated and don’t really match the modern design of nowadays. That’s why we decided to keep up with the times and create more great new looking themes for you which will definitely be working with latest WordPress versions. Maybe its time for you to upgrade your website with one of our new themes? We will keep providing best support for our customers and even for old themes but with appropriate WordPress version. We appreciate your understanding regarding this matter and thank you for choosing our themes!