Altering a column: null to not null
The syntax you used is from a SQL Server example, not from MySQL. It would be good to check the MySQL documentation about ALTER TABLE syntax.

In MySQL, the ALTER COLUMN subclause can only be used for setting or dropping the default value of the column (SET DEFAULT literal or DROP DEFAULT).

You need to use either CHANGE COLUMN (note that column name is doubled as you could use this to change its name):

ALTER TABLE MyTable  
    CHANGE COLUMN comment comment BIGINT NOT NULL;


or MODIFY COLUMN:

ALTER TABLE MyTable 
    MODIFY COLUMN comment BIGINT NOT NULL;


Now, there are 2 more problems, beside the syntax:


Seeing the CREATE TABLE, converting a "comment" column from TEXT to BIGINT does not make much sense. I'll assume that you want to keep it TEXT and only make it NOT NULL.
There might be NULL values already in the table. If that's the case, the statement will fail with errors. So, you have to first UPDATE those values to some non-null value (say the empty string) and then modify the column:


So our statements become:

UPDATE glpi_ticketsatisfactions 
SET comment = ''
WHERE Comment IS NULL ;

ALTER TABLE glpi_ticketsatisfactions 
    MODIFY COLUMN comment TEXT COLLATE utf8_unicode_ci NOT NULL ;