We need more details. You haven't told us what you tried.

• You must use UEFI BIOS (not Legacy BIOS) TPM must be enabled, the C: drive must be formatted as GPT.
• With a flash drive at least 8 GB size, use Microsoft's Media Creation tool to create the Windows 11 installer.
• Disconnect all peripherals: printers; USB devices; media cards; USB hubs and extenders; etc.
• Insert the flash drive. Immediately after pressing the power button to boot the laptop (or if you're rebooting the laptop), press F12 repeatedly to enter the One Time Boot Menu and select the installer flash drive.

What type of problem are you having with the installation? Are you seeing an error or message that you either do not have Secure Boot or TPM enabled (they must be enabled in the BIOS)? Your system does support Windows 11 and Dell does offer Windows 11 drivers for the 5481. Are you trying to update or have you gone to the Windows 11 site here? At that site you can either run Installation Assistant which is the update procedure. Or you can download the media creation tool to create a bootable Windows 11 installation flash drive.