Hidden User Accont

Hidden user Account - is a way to restrict who can "forward" messages. When this setting is enabled, forwarded messages will no longer lead back to author's account. They'll just display an unclickable name in the “from” field.

https://telegram.org/blog/unsend-privacy-emoji#anonymous-forwarding

In other words, in the settings of Telegram account there is an option to "Hide user's ID during Forwarding" of the message. This setting makes it impossible to obtain user's ID from his forwarded message.

Setting can be changed only by user himself in:
Settings > Privacy and Security > Forwarded messages
Set to "All" - to show user ID in forwarded message.

If you try to use the message of such "Hidden" user in the bot (to get his Balance or variables details) - you will get an error. In groups, if you try to reply to the message of a "Hidden" user you MAY get an error as well.

Read more about Tags in Message, to comfortably deal with users whose accounts are hidden.