This could be due to UNICODE Characters present in the message due to adding / copying it from an outside source.
Copy Pasting from a software program or a website can bring these special characters into your message.
However, try copying the message into - Notepad (pre-loaded on most Windows computers) which will strip out unicode characters and then copy/paste into the SMS platform's message box.
Need more info on Unicode Characters? Click here: UNICODE
NOTE: Unicode messages will be counted as 1 message unit per 70 characters instead of 160 characters for a standard text message.