Convert TXT to CRYPT

You cannot just simply create WhastApp archive (.crypt) file from any random .txt ile, even if its the one exported from WhatsApp. This is because such .txt file contains only one single exported conversation without any formatting, whilst CRYPT format is a full message history backup (compressed HTML format).

You have to look for the CRYPT/CRYPT/CRYPT12 etc archives on your Android device or msgstore.db file on iOS devices if you want to attempt to make a CRYPT archive from them. In such case refer to individual conversions like crypt12 to crypt conversion etc.

Simply put, it is impossible to convert TXT to CRYPT directly.
Thus, there is no so-called txt to crypt converter or a free online .txt to .crypt converter to be found.