Catalog Search indexer process unknown error: There is no such engine: mysql2

Run this query

UPDATE core_config_data set value = 'mysql' where path='catalog/search/engine';

And run Following commands

php -f bin/magento setup:upgrade
php -f bin/magento cache:flush
php -f bin/magento indexer:reindex

Check your config in the node - catalog/search/engine, by default it must be MySQL, unless you are using some other engine. In your case, for some reason, it set to mysql2, but Magento can't find any information about the type.

catalog/search/engine can be found in the core_config_data database table as a path.