its under Advanced Mode
Press Setup Mode switch or F7 function key can switch between EZ Mode and
Advanced Mode in BIOS setup.
secure Boot Support [Disabled]
Enables or disables secure boot support.
[Enabled] Enables the secure boot function and allow you to set the secure
boot settings.
[Disabled] Disables this function.
▶Secure Boot Mode [Standard]
Selects the secure boot mode. This item is to select how the secure boot keys be
loaded. This item appears when Secure Boot Support is enabled.
[Standard] The system will automatically load the secure keys from BIOS.
[Custom] Allows user to configure the secure boot settings and manually load
the secure keys.
your Manuel
https://www.msi.com/product/motherboard/support/Z170A-PC-MATE.html#down-manual
Custom] Allows user to configure the secure boot settings and manually load
the secure keys. would be the best option