The error occurred because my bot wasn't an admin in the channel specified in the reply_parameters. According to the Telegram Bots Documentation, all bots, regardless of settings, can only access messages from:

1. Private chats: Only when a user initiates the chat by starting it.

2. Groups / Supergroups:

• Privacy Mode = Enabled:

  • Only messages explicitly addressed to it (e.g., mentioning the bot or using its commands)

• Privacy Mode = Disabled:

  • All messages in the group, but this requires explicit permission from a group admin.

NOTE: Privacy mode is enabled by default for all bots, except bots that were added to a group as admins. Read more

3. Channels: Only where the bot is an admin.