It will refuse by default because the system has an unsupported CPU.  While there are ways to bypass this, note that Microsoft has officially stated that there may come a time when the CPU support is enforced - at which point, Windows 11 will cease to update.

I installed Windows 11 on my E5430 which is an older system and the installation went without a hitch. Have you tried downloading the media creation tool here and clean installing from the bootable flash drive file that is created? And this page at How-To Geek has more information. And this page at MS has information on how to install on unsupported hardware.

Also this YouTube tutorial will help.

Thanks JOcean and ejn63 for your answers.

Would you know why this CPU is not supported? Intel Core i5-6300U 2,4-3.0GHz is fast enough, not that old... I noticed there were workarounds, but I found no information about the risk I'm taking if I use them. Any hints?

FYI - looks like there is an official method to upgrade the TPM module to 2.0 for these older Dell Laptops (which will help meet some of the Win11 min requirements).  ie for E5470

https://www.dell.com/support/home/en-au/product-support/product/latitude-e5470-laptop/drivers

Note it may still fail the list of supported CPUs, but as mentioned above this check can be side stepped (using the not recommended approach).

https://support.microsoft.com/en-us/windows/ways-to-install-windows-11-e0edbbfb-cfc5-4011-868b-2ce77ac7c70e