You have mandatory questions in the survey, but you think that the message "Please fill in the missing answers" may be misleading for your respondents?
You can change it for a single survey / test.
There is an option in the survey settings (Settings --> survey settings --> Messaging
The message about the necessity to complete the obligatory answers
By selecting this option, you can change the content of the message displayed to the respondents when they do not answer the mandatory questions in the survey.
Click it and enter your own text.
It's ready :)