Can someone help me with this problem, I have tried to figure it out for hours

[Philip32]

 

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Error: Call to undefined function RSThemes\Service\str_slug() in /var/www/vhosts/zap814793-1.plesk08.zap-webspace.com/httpdocs/whmcs/modules/addons/RSThemes/src/Service/Hooks.php:73
Stack trace:
#0 /var/www/vhosts/zap814793-1.plesk08.zap-webspace.com/httpdocs/whmcs/modules/addons/RSThemes/hooks.php(128): RSThemes\Service\Hooks->ClientAreaHomepagePanels()
#1 /var/www/vhosts/zap814793-1.plesk08.zap-webspace.com/httpdocs/whmcs/vendor/whmcs/whmcs-foundation/lib/Hook/Manager.php(0): WHMCS\Utility\SafeInclude::{closure}(Object(WHMCS\View\Menu\Item))
#2 /var/www/vhosts/zap814793-1.plesk08.zap-webspace.com/httpdocs/whmcs/vendor/illuminate/support/Facades/Facade.php(261): WHMCS\Hook\Manager->run('ClientAreaHomep...', Array, true)
#3 /var/www/vhosts/zap814793-1.plesk08.zap-webspace.com/httpdocs/whmcs/includes/functions.php(0): Illuminate\Support\Facades\Facade::__callStatic('run', Array)
#4 /var/www/vhosts/zap814793-1.plesk08.zap-webspace.com/httpdocs/whmcs/clientarea.php(0): run_hook('ClientAreaHomep...', Array, true)
#5 {main}

[steven99]

It can't find the function str_slug() within the RSThemes\Service\ namespace most likely because RSThemes\Service is not loading via a autoloader.   If there is a RSThemes/Service.php file, that has to be included in some way in to the hooks file. 

[pRieStaKos]

I think the developers of the theme could help you more.