The common reasons that cause you to “Fatal error: Uncaught Error: Call to undefined function” are:
- The host doesn’t meet these requirements.
- PHP function not found in the current version of the WordPress.
- PHP function not found in the theme, plugin, or third-party script where the referenced file depends to.
You can’t access the site’s dashboard (/wp-admin) when this error presents on your screen.
I don’t recommend you to update the WordPress version because the basic installation of WordPress with default theme and plugins free won’t produce this error if your host meets these requirements. Updating WordPrsss should always be the last option for this case.
As this error always mentions the affected file, you can easily get the source of the problem in the specific file that is mentioned.
If the screen shows a file path to a theme like wp-content/themes/themefolder, the trouble is with that theme. If it shows wp-content/plugins/pluginfolder, the trouble is with that plugin.
Access your site with your FTP or SFTP account and locate to the themes or plugin folder. Then rename the mentioned folder of the theme or plugin in the error notice to quickly disable it. For example, if the folder name is awesomeplugin; rename it to awesomeplugin_disabled or anything else preferred. By doing this, your site will be recovered. Your task is now to contact the theme or plugin author to get the best advice on how to deal with it. So you can decide whether you will ask him to fix the error or use other alternative plugin or theme.