It seems like they may have finally fixed this! At least on PC, as of version 2023.23.10.440.985016.
Try your colorless brawl decks again, and let me know I'm crazy if they don't work.

Previously I had drafted an Ulamog historic brawl deck that I'd been trying to add 24x Wastes to, and would get a network error when trying to save. To "save and exit" the deck builder, you'd have to get the error, then manually back out to a different screen (i.e. Home, or Decks), and it would have saved the deck but show as invalid. Because of these issues, I'd been very hesitant to spend any wildcards on the missing pieces. Now, it doesn't show invalid anymore, so now I'm just torn between spending my wildcards on this or on new standard-legal cards XD

That having been said, they've re-broken things they've fixed before with later updates, so who knows if it'll stay this way.

i hope this will be fixed

Fix this

STOP TRYING TO MAKE ARENA INTO EVERY OTHER DIGITAL CARD GAME FOR 5 MINUTES AND FIX THIS ISSUE WITH A BASIC LAND

A single Basic Land has more issues than cards with seek/conjure/draft/perpetual effects

Can't make a brawl deck with colorless commander and waste land. one waste land added and deck will not save will save if no wastes are in deck. I spend all my card token thing to make a colorless deck now I has no card tokens and I has no colorless brawl deck. Thanks for wasting my time in the most horrible ways and taking all my stuff while doing it.

The issue is that they do not have the basic land type set to them. It's that simple. So whatever rule they use for matching identities is failing even though the colour identity (or lack thereof) is the same.

12/20/2022 Still an issue. Still no reply from the Arena team.

12/18/2022 Still an issue. Still no reply from the Arena team.

12/16/2022 Still an issue. Still no reply from the Arena team.

Facing the same issue.

having same issue

I've been looking forward to this for a while, and i'm disappointed in there being no recognition, or update on this issue

ARENA CUSTOMER SERVICE PROCESSES CREATED BY THE 'B' TEAM, UNFORTUNATELY.

12/15/2022: Still no resolution for what is likely a straightforward programming fix (issue above).
12/15/2022: Still no direct customer support for bugs and errors from Arena. Everything is thrown onto this page, with no transparency and exactly one prioritization method.

Arena team is so worthless on this kind of thing. They have no good feedback system; there's no way people are actually monitoring this with 9 votes so far, and they have no way to contact them elsewhere.

I'm facing the heckin same problem over here. At first was very excited for having the Wastes finally arriving to Arena, but being unnable to use them is very frustrating. Hope they fix this asap.