I need more information about the errors.
1) Can you please copy & paste the error messages?
2) If there's no error message, can you attach a screenshot?
3) What is the control panel of the hosting? DirectAdmin? CPanel? Or something else?
In the mean time, here's several suggestions (since I don't have the exact error messages):
1) Download the ZIP file from
https://www.schlix.com/html/schlix-cms-downloads.html and then extract it. Then upload ALL the files in /schlix/ via FTP or SFTP to the ROOT folder of SCHLIX CMS installation (you have to go inside the folder /schlix). Use FileZilla or some FTP/SFTP client if it makes it easier.
2) Go to your hosting control panel, select the PHP version to v7.4. Then upgrade manually.
Let me know if it still doesn't work (and please give me the error message), I'll try to provide assistance as much as possible.