In version 4.25.12 (released version 126.96.36.199) we added FULLTEXT indexes to some columns in the database to speed up searches and filtering by tags or by customer groups.
If you are getting this error during update:
it means that you are using an older version of database.
The fulltext indexes are supported by MySQL from version 5.6 and by MariaDB from version 10.0.5.
You might want to check out more information about LiveAgent requirements.