Open the script in your local JMeter and do a search for characters that don't match the ASCII table. Since ASCII characters are in the range of hexadecimal 00 - 7F, look for any characters outside that range and make sure to check the Regular Exp. checkbox:
Once found, get rid of all non ASCII characters, save the file and upload it to the Blazemeter test again. The error message will no longer show up.
The entire script is parsed, so all components, even the disabled ones, need to contain ASCII characters only.