Firstly, make sure that Secure Boot has been disabled. For most PCs, you can disable Secure Boot through the PC's firmware (BIOS) menu. You can do so by following the steps below:
1. Open the PC BIOS menu. You can often access this menu by pressing a key during the bootup sequence, such as F1, F2, F12, or Esc.
2. Find the Secure Boot setting, then set it to Disabled. This option is usually in either the Security tab, the Boot tab, or the Authentication tab.
3. Save changes and exit. The PC reboots.
Secondly, also check that Bitlocker has been turned off. You can do so by following the steps below:
1. Click Start, click Control Panel, click System and Security, and then click Bitlocker Drive Encryption.
2. Select the drive on which you want Bitlocker turned off, and click Turn Off Bitlocker, then click Decrypt the volume.
3. Bitlocker will then be turned off.
If the boot menu is still not seen after disabling Secure Boot, uninstall Remix OS for PC, restart your PC and reinstall Remix OS for PC again.