Did you put exactly 16 characters in your encryption string?
Try never to answer your own questions but edit your original question or put a comment. If at the split Is encrypted value the microflow goes to the log message your original string is not encrypted and you have to look at the cause why this string might not be encrypted. If you get another error please give us the error so we can help you further.
From the error below it seems that your email message object is not correct. Set a breakpoint just before sending your mail and double check the contents of your mail message.
Yes,I have put exactly 16 characters.
Are you sure the correct parameters are provided in the Java action call Encryption.DecryptString? MF in modelshare shows:
value (not set)
key (not set)
prefix (not set)