HD disk is encrypted, I can't re-install MacOS, have any idea to solve it

Welcome to the Apple Support Communities.

See: Use FileVault to encrypt the startup disk on your Mac - Apple Support, specifically the "Reset password" section.

Jack

The default encryption password for your Boot drive for most users (those who just checked a checkbox on FIRST RUN to encrypt their drive) is the same as their initial login password created at FIRST RUN.

^--- If you deliberately encrypted your drive later, the above does not apply to you.