admin/environment/php extension/soap
From MoodleDocs
SOAP extension for PHP
The optional SOAP extension is useful for web services and some plugins.
Installing
Debian
# apt-get install php-soap
CentOS
Download and install soap:
dnf install php-soap
You might need to restart PHP-FPM to apply the change in configuration:
systemctl restart php-fpm
If you are unable to find the correct package try searching with:
dnf search 'php*-soap'
Microsoft Windows
1) Locate php.ini e.g. for XAMPP: C:\xampp\php\php.ini
2) Search for the string inside the quotes - ";extension=php_soap.dll"
3) Remove the semicolon, save the file and restart apache.