FTP user name & password not work when using FileZilla

which package are you using for ftp on lightsail instance - vsftpd? or another please check the configuration first

for example if the package is vsftpd

Uncomment the line local_enable=YES to allow local users to log in.
Uncomment the line write_enable=YES to allow users to write to the server.
Uncomment the line chroot_local_user=YES to lock users in their home directories.

Did you activate FTP passive mode: see https://support.tigertech.net/filezilla-passive

is it necessary for an SSL certificate configure vsftpd?

SSL is separate from userid/ password. If you're happy with the working keypair, you can skip this.

Thanks for your support.

Actually, I made a mistake when creating an instance. After making some changes in security groups, it works.