You may have to do a BIOS update.  However, there should be a Secure Boot option whether it's already there or requires a BIOS update.  Have a look here:    17 and 18 images down. You would have to set your OS to Windows WHQL under Secure Boot mode.

Just be aware that BIOS changes can result in a non-bootable system.  This is the warning given by EA themselves after they've decided to force people into making such changes.  I have enabled Secure boot on my Asus ROG Strix Z790-F with no issues thankfully.

https://pcper.com/2014/05/msi-z97-gaming-7-motherboard-review/6/

How to Enable Secure Boot on MSI Z97-Gaming 7 – Complete Guide

Applicable For: MSI Z97-Gaming 7 Motherboard (UEFI BIOS)
Difficulty: Intermediate

📌 Introduction

Secure Boot is a security feature in UEFI that ensures only trusted operating systems and firmware load during the boot process. Enabling Secure Boot is essential for Windows 11 compatibility and enhanced system security.

This guide will walk you through enabling Secure Boot on an MSI Z97-Gaming 7 motherboard.

⚠️ Prerequisites

Before proceeding, ensure:
✅ Your system is running in UEFI mode (not Legacy/CSM).
✅ Your OS (Windows 10/11) is installed in UEFI mode.
✅ Your BIOS is updated to the latest version (check MSI Support).
✅ You have backed up important data (BIOS changes carry risks).

🔧 Step-by-Step Guide

1. Enter BIOS Setup

1. Restart your PC.
2. Repeatedly press DEL or F2 during boot to enter BIOS.

2. Switch to UEFI Mode (If Needed)

If your system is in Legacy/CSM mode, you must disable it first:

• Go to Settings - Advanced - Windows OS Configuration.
• Set "Windows 10/11 WHQL Support" to Enabled.
• Set "Boot Mode Select" to UEFI.
• Disable "CSM" (Compatibility Support Module).

3. Enable Secure Boot

1. Navigate to Settings - Advanced - Secure Boot.
2. Set "Secure Boot" to Enabled.
3. Select "Secure Boot Mode" - Standard (for Windows).
4. If prompted, load factory defaults or install default keys.

4. Save & Exit

1. Press F10 to save changes and exit.
2. Confirm with Yes.
3. Your system will reboot with Secure Boot enabled.

🔍 Verification

To confirm Secure Boot is active:

• Windows:
• Open CMD as Admin - Run:

Confirm-SecureBootUEFI

• If it returns "True", Secure Boot is enabled.
• BIOS:
• Re-enter BIOS - Check Secure Boot Status under Settings.

❌ Troubleshooting

Issue: "Secure Boot Not Available"

• Ensure CSM is disabled and Windows WHQL Support is enabled.
• Update BIOS if the option is missing.

Issue: "Invalid Signature" or Boot Failure

• Reset Secure Boot keys to "Restore Factory Keys" in BIOS.
• Reinstall OS in UEFI mode if necessary.

Issue: Dual Boot (Linux/Other OS) Problems

• Some OS require manual key enrolment - Check your distro’s documentation.

📢 Final Notes

• Secure Boot enhances security but may cause issues with unsigned drivers/OS.
• If you face boot problems, disable Secure Boot temporarily.
• For further help, visit the MSI Support Forum.

hi,

Thank you for your explanations and all this information. Unfortunately security boot remains inaccessible (option in gray).

Secure Boot Mode = Custom

I tested in custom, still security boot inactive

Have a look here. Yes the BIOS view in this example is of the older type but the suggestions are valid. 

https://www.makeuseof.com/secure-boot-grayed-out-bios/

To consider:

Reset your BIOS to Optimised / Factory Defaults - Sometimes you just have to default things.

Set an Administrator password. Interesting. If you DO,  make it a very simple one  just for the purpose of hopefully getting Secure boot to be enabled. 

Update your BIOS. Trust me, the amount of people on VERY old revision BIOS is huge. Updating the BIOS can be a serious worry for many but there are a LOT of fixes to be gained.  I recently updated my BIOS to fix a CPU Code fault with Intel i9 CPUs. This was causing me NO end of problems with BF but 10 mins later after a BIOS update, all my BF problems went away.

If you choose to update your BIOS. Read ALL the instructions carefully. It's very simple to do but not reading instructions can be problematic.

colder1971​ If you follow step by step this guide i believe it is working

Fantastic news. 

That is brilliant. Kudos for getting it done (and thank god it didn't screw up your system boot 😄)

Happy gaming! 🙂

Incidentally, what process did you go through to get it sorted in the end?  (helpful to others with the same issue).

sure,

I updated the bios (done since Sunday) . Install the default settings  . Disable fast boot in BIOS. I created an Admin Password. I set Secure Boot Mode = Custom. and secure bot support = enabled. 

After each step I restarted the bios saving.