This happen in BOT Inboxer when the page is already enabled by another Facebook account or another system user in the your system.
To solve this & enable from where you are trying is to find out the account in the system where this page is already enabled. You will need to delete the BOT from BOT Settings Menu from that account. Then you can enable it again for new account.
If for any reason you can’t find the account where it is actually enabled already, you can check in database. Though it’s not recommended if you don’t have enough expertise in database. You will get information about the account id from where it is enabled from this table messenger_bot_page_info
Then from that account you can delete the bot.
If finally you can’t login that account, you can modify the database by searching the page. To remove the bot from that page by modifying table messenger_bot_page_info , Column Name : bot_enabled = 0