Hi, sorry you’re having difficulty. This is the heart of your error message:
MySQL server has gone away
This means your MySQL server crashed and restarted in the middle of rekeying the wp_usermeta table. Do you have many users? With lots of metadata each?
The rekeying operation requires temp space on your hard drive, enough to hold the table being rekeyed with its indexes. MySQL really doesn’t like it when the temp space runs out, and restarts. And, on some Linux installations the /tmp directory is a ramdisk to make things fast, so it runs out quickly.
So, try tweaking your MySQL configuration. See this.
https://dev.mysql.com/doc/refman/8.0/en/server-options.html#option_mysqld_tmpdir
As root on your MySQL machine edit the /etc/mysql/mysql.conf.d/mysqld.cnf file.
Near the line where it says datadir = something/or/other add a line saying
tmpdir = /var/tmp
then restart your server by saying
sudo service mysql restart
Please let me know if this was the problem.
By the way if you do have many users check out the companion plugin. https://wordpress.org/plugins/index-wp-users-for-speed/