The problem is probably related to your MySQL (or MariaDB) configuration. To be exact and as described here dev.mysql.com/doc/refman/5.7/en/innodb-limits.html if "innodb_large_prefix" is set to false, this limit is imposed to databases with utf8mb4_unicode_ci collation.

Your hosting company may have set this option explicitly for backwards compatibility reasons (the default is enabled).

The other option is to download and extract the K2 zip file and edit the file /administrator/components/com_k2/install.mysql.sql replacing all instances of "CHARSET=utf8mb4 DEFAULT COLLATE=utf8mb4_unicode_ci" with "CHARSET=utf8 DEFAULT COLLATE=utf8_general_ci". Then repackage the contents you extracted into a new zip file and finally install that file in Joomla.