SOLUTION:
$ sudo mysql -u root -p
MySQL comm. [(none)]> use nextcloud;
MySQL comm. [nextcloud]> set global innodb_large_prefix=on;
MySQL comm. [nextcloud]> set global innodb_file_format=Barracuda;
$ sudo -u www-data php occ maintenance:repair
SOLUTION:
$ sudo mysql -u root -p
MySQL comm. [(none)]> use nextcloud;
MySQL comm. [nextcloud]> set global innodb_large_prefix=on;
MySQL comm. [nextcloud]> set global innodb_file_format=Barracuda;
$ sudo -u www-data php occ maintenance:repair