What generation of Intel CPU do you have? I ask because it would appear that none of the pre-Generation 8 CPUs will be supported.

I have a core i5-2500

So even if I do a bios update ,that won't add secure boot?

Exactly but even if it did it does not matter. Your CPU is not going to be supported in Windows 11. It is far too old.

@amiribrahim The list below shows the only Dell OptiPlex PCs that meet the specs for Windows 11.  You'll need an 8th gen. Intel processor (e.g. i7-8700) or better for Windows 11 or similar gen. AMD processor.

OptiPlex

The following list of Dell OptiPlex computers that have been tested for upgrade to Windows 11. If your computer model is not listed, Dell is not testing the device, and drivers have not been updated for that model.

• OptiPlex 3060
• OptiPlex 3070
• OptiPlex 3080
• OptiPlex 3090 Ultra
• OptiPlex 3280 All-in-One
• OptiPlex 5060
• OptiPlex 5070
• OptiPlex 5080
• OptiPlex 5090
• OptiPlex 5260 All-in-One
• OptiPlex 5270 All-in-One
• OptiPlex 5480 All-in-One
• OptiPlex 5490 All-in-One
• OptiPlex 7060
• OptiPlex 7070
• OptiPlex 7070 Ultra
• OptiPlex 7071
• OptiPlex 7080
• OptiPlex 7090
• OptiPlex 7090 Ultra
• OptiPlex 7460 All-in-One
• OptiPlex 7470 All-in-One
• OptiPlex 7480 All-in-One
• OptiPlex 7490 All-in-One
• OptiPlex 7760 All-in-One
• OptiPlex 7770 All-in-One
• OptiPlex 7780 All-in-One
• OptiPlex XE3

I have 11 to work on Dell Optiplex 790

Solution is here. View and read carefully.
https://www.youtube.com/watch?v=d2B4tNEhi5M

Hi, My CPU is i5-2400 Intel @ 3.10 Ghz. Then I have the Nvidia Ge force GT 710 and a Samsung SSD 500 evo. Someone this morning in Facebook said that I could install the Insiders Dev windows 11, and so I did. In the link I gave there is a cheat that they do replacing a .dll
It works. 

@Nikos Maraslis  Advance, Windows Insider, versions of Win 11 are able to run on older hardware.  But Microsoft is still sticking with Intel 8th gen. CPU or newer when they formally release Windows 11, at which point all Windows Insider pre-release versions running on unsupported hardware will cease to work.  Microsoft may vary the requirements - they are evaluating the operation of Windows 11 in the Dev. Channel running on Intel 7th gen. and AMD Ryzen 1st gen. CPUs and may include these in the final approved hardware specification.  Processors below these have been ruled out.

This is in Greek. If you can translate it might help. Share it please:

https://o-lathranagnostis.blogspot.com/2021/10/windows-11-pc.html 

Dell 790 Optiplex