This pesky error when upgrading Moodle typically indicates that you're using a database engine type in Moodle that's no longer supported.

Most commonly, it refers to a scenario where you're using a database engine such as MyISAM. In the later versions of Moodle, some of these legacy database engines have been phased out from the supported engine types.

One of the best and most reliable database engines for MySQL is InnoDB, and in 2020 you should most certainly be using InnoDB above MyISAM.

The most reliable means of resolving this warning is to perform the following:

1. Put your Moodle site into maintenance mode.

2. Via the command line (SSH), CD into the Moodle root directory, using a command such as:

   cd /var/httpdocs/moodle/public_html

3. Then, run the following command:
   
   php admin/cli/mysql_engine.php

4. This will output the relevant suggested command for Moodle's MySQL database conversion tool. You'll then typically want to enter something like the following into the command line:
   
   sudo -u www-data /usr/bin/php admin/cli/mysql_engine.php --engine=InnoDB
   
   Where www-data refers to the Apache user, and InnoDB refers to the database engine you wish to convert the current database to.

And viola! You're now good to go.