Check this post: https://gvectors.com/forum/postid/31990/

Checked your website, and sent you a Pm message. I don't see any Issue with PM

1. Switch to default theme.

2. Disable all plugins except wpForo and the addon.

3. Enable WP Debug to a log.

4. Disable for a while your CDN.

And as Chris posted above, there is a specific forum for addons, maybe they can help you more.

What happens if you switch for a while to the default theme?

The theme developer respond as follows

I checked your Project LightSpeed and i see that you were able to disable it already. - https://share.getcloudapp.com/lluE49Wm

Please also note that our Project LightSpeed tool is for our Thrive files & script only and it doesn’t affect the files or code of your app or any 3rd party plugin.

If you experience issues with that app or 3rd party plugins, I would recommend you to contact their support or developer instead for assistance.
________

Following this responce:

I set the site to the default WordPress theme. and the error still persists as shown here:

https://share.getcloudapp.com/BluoNKpv

I found another thread in this forum with the same issue but that thread ended without a resolution.

@dimalifragis  

I have removed all caching and, even turned server caching.

issue still present.

Please read carefully what i wrote. Not a caching issue but some Litespeed script.

SEE THE html source of YOUR page:

<script type="text/javascript">
			window.flatStyles = window.flatStyles || ''

			window.lightspeedOptimizeStylesheet = function () {
				const currentStylesheet = document.querySelector( '.tcb-lightspeed-style:not()' )

After registration, I can't Log in your website, need to check from where come that JS error.