You can e.g. use phpmyadmin or the mysql command line tool to add the missing index. Aunt G. will tell you the exact syntax if necessary. This is how the mentioned table should look like afterwards:
MariaDB [nextcloud]> describe oc_calendarobjects_props;
+--------------+---------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+--------------+---------------------+------+-----+---------+----------------+
| id | bigint(20) unsigned | NO | PRI | NULL | auto_increment |
| calendarid | bigint(20) | NO | MUL | 0 | |
| objectid | bigint(20) unsigned | NO | MUL | 0 | |
| name | varchar(64) | YES | MUL | NULL | |
| parameter | varchar(64) | YES | | NULL | |
| value | varchar(255) | YES | MUL | NULL | |
| calendartype | int(11) | NO | | 0 | |
+--------------+---------------------+------+-----+---------+----------------+