Fatal error after updating plugins

Hi, I have been tasked with looking after a group’s website after the creator left and I have no clue about WordPress, I usually use WYSIWYG programs. I updated the WordPress version which was out of date and that went fine. I saw on the dashboard all of the plugins were out of date, and the updates were compatible with the latest WordPress so I clicked on Update All and am now getting the following error with the website. Please can anyone give me some pointers on how to restore the site’s visibility? The site is [ redundant link removed ]
Thank you so much for any help!

Fatal error: Uncaught Error: Call to undefined function trailingslashit() in /homepages/24/d243408606/htdocs/clickandbuilds/MedicalArtistsAssociation/wp-includes/class-wp-textdomain-registry.php:103 Stack trace: #0 /homepages/24/d243408606/htdocs/clickandbuilds/MedicalArtistsAssociation/wp-includes/l10n.php(784): WP_Textdomain_Registry->set('default', 'en_GB', '/homepages/24/d...') #1 /homepages/24/d243408606/htdocs/clickandbuilds/MedicalArtistsAssociation/wp-includes/load.php(1401): load_textdomain('default', '/homepages/24/d...', 'en_GB') #2 /homepages/24/d243408606/htdocs/clickandbuilds/MedicalArtistsAssociation/wp-includes/load.php(278): wp_load_translations_early() #3 /homepages/24/d243408606/htdocs/clickandbuilds/MedicalArtistsAssociation/wp-settings.php(74): wp_maintenance() #4 /homepages/24/d243408606/htdocs/clickandbuilds/MedicalArtistsAssociation/wp-config.php(80): require_once('/homepages/24/d...') #5 /homepages/24/d243408606/htdocs/clickandbuilds/MedicalArtistsAssociation/wp-load.php(50): require_once('/homepa in /homepages/24/d243408606/htdocs/clickandbuilds/MedicalArtistsAssociation/wp-includes/class-wp-textdomain-registry.php on line 103

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