The channel ID in .env or tokens.txt doesn't resolve to a real channel the bot can see.
Typo in the ID, the channel was deleted, the bot account isn't a member of the server, or 0 was used but no default channel is configured.
Enable Developer Mode in Discord, right-click the target channel, Copy ID, and paste it into .env. Make sure the selfbot account is a member of the server and can see that channel. If using 0, configure a valid default channel first.
TOKENS="token 999999999999999999"TOKENS="token 1234567890123456789"