Are you using Samsung Messages, Google Messages, or something else? Can you show us a screenshot of that exact error message? Make sure to block out any identifying information before you post it.
The only reasons your text would be converted to MMS would be if you're sending media (i.e., a photo or video), if it's a group text, or if the message exceeds 160 characters. (I think emojis might convert it to MMS as well.)