Sometimes Telegram can give the user a 'chat not found' error. This is caused when the Telegram session file gets corrupted, or if you change the description of the channel in Telegram's settings.
To fix the error, first close the Telegram app. Then, locate the Telegram Session file on your computer's C drive.
Delete the file 'Telegram Session'.
Then you will need to uninstall Telegram FX Copier, and reinstall it. Follow the instructions here: https://helpdesk.telegramfxcopier.io/367085-I-already-have-a-previous-version-Should-I-uninstall-it