Facing problems with my personal Blog

I have received an email from WordPress about an error on my blog. In effect, my last 3 articles appears on the main page but when clicking on them, there is an error message :

The error is : Uncaught Error: Class ‘Allegiant_Notify_System’ not found in /home/mtptomtlpc/www/wp-content/themes/activello/inc/welcome-screen/sections/actions-required.php:91

My config :
WordPress version 5.5.1
Thème actuel : Activello (version 1.4.1)
Extension actuelle : (version )
PHP version 7.2.31

Thank you so much for any help.

That’s not our theme and not even the WordPress