Hi user067,
Thank you for posting in the Microsoft Community Forums.
Contact an administrator:
If you're using a Chromebook at a school or company, the best thing to do is contact your network administrator or IT support. They may be able to provide you with instructions on how to change your wallpaper, or may be able to adjust their policies to allow you to customize your wallpaper.
Check user accounts:
Make sure you're using an administrator account. In some cases, regular user accounts may not have enough permissions to change system settings, including wallpapers.
Use Developer Mode (proceed with caution):
If your Chromebook supports it and you understand the risks that may come with turning on Developer Mode (e.g., loss of warranty, loss of data, etc.), you can try entering Developer Mode. This usually allows you to bypass some administrative policies, but it can also make your device more vulnerable to malware.
Turn on your Chromebook and press Esc + Refresh (F3) key + Power key to reboot your device and enter recovery mode.
When “Chrome OS is missing or damaged” appears on the screen, press Ctrl + D to enter Developer Mode.
Note that this will reset your device and may require you to sign in to your Google account again.
Best regards
Neuvi