Looking over the site i can’t see what would cause a CLS failure – there are many tiny CLS events being triggered but those should not accumulate to a fail.
For the Mobile Menu you may want to enable the Off Canvas panel as its better suited Longer menus and will trigger less tiny CLS events:
https://docs.generatepress.com/article/off-canvas-panel-overview/
Optimization wise:
You have a few Image files that are fairly large in size – for example the Million Pound Betting and Trading image is over 120kb – and there are quite a few other images of a similar size. You should optimize them and better to use JPEG as it will afford you smaller files.
The Latest Posts page you have long excerpts… you may want to consider limiting these to a small text excerpt – its a lot less HTML and content to be loaded.
I am also seeing a YouTube Javascript file being loaded – if you’re adding YouTube videos then a plugin like this will defer loading the scripts and video until the user clicks the video:
https://wordpress.org/plugins/wp-youtube-lyte/