It might be an issue with the PHP version I would suggest you update the server PHP version to 7.4 or 7.4 plus as per our version compatibility page. I would suggest you contact your hosting provider to upgrade the PHP version. It is also recommended by WordPress as well. Please have a look at this article.
I would also recommend you if you have access to your php.ini file look for the disable_functions item and remove ini_set from the list. Depending on your server setup you may also need to restart Apache for this to take effect. If you do not have access to php.ini you will need to ask your hosting provider to make the change.
If it doesn’t work for you after setting the server configuration, please share your details in a secure note. Please provide the following details
WordPress Login URL
Admin level username and password
You can find the Secure Note button at the bottom of your posts