Your system is not compatible because you have a 7th generation CPU and MS specifies 8th gen as the oldest supported (there are two 7th gen CPUs included but they are in very specific systems). This is not  a Dell decision, this was decided by Microsoft. And there are a number of manufacturers that find themselves in the same situation. Would Windows 11 run on your system, probably. But if you do manage to get it installed and running, and need Dell tech support down the road, Dell tech support will insist that you reinstall Windows 10 before they will offer assistance since you are not on the Dell compatible list.
Yes the 2017 XPS 8920 has TPM2 capability in the BIOS. But DELL is not including the 8920 in their WIN 11 upgradeability for some reason. I ran the WIN 11 system checker, said I needed TPM2 and that my CPU was not compatible. So I reboot my 8920, pressed F2, BIOS popped up. Was able to turn on the TPM in the security settings. Took 2 reboots before my computer booted normally. Ran the WIN 11 checker again, TPM2 all good, CPU still not compatible. WIN 11 specs say 1 Ghz or faster. I have a Intel 3.6 Ghz processor. Not sure at this point what's up with that. Just not happy with DELL that my 8920 is not included in the WIN 11 upgrade testing.
XPS 8920, Windows 11 specs, TPM2 and CPU