'Illegal mix of collations (utf8_general_ci,IMPLICIT) and (latin1_swedish_ci,EXPLICIT) for operation '='' on query
NAME_CONST is for internal use and has no possibility to convert text or use a collation
use the conventional way with CAST or CONVERT
UPDATE site_user_master 
SET 
    is_active = 'N',
    modification_date =  1615022896
    ,passkey =  CONVERT(_latin1'9419' USING latin1_swedish_ci)
WHERE
    email_id = CONVERT(_latin1'Raj.fm.kangra@gmail.com' USING latin1_swedish_ci) 

Also it is not a good practrice to save passwords in plain text, there are a lot of function to hash the password https://dev.mysql.com/doc/refman/8.0/en/encryption-functions.html