Turnitin has minimum and maximum limits on the number of words, characters and pages you can submit. If you go below the limit, you will see the error Your submission must contain 20 words or more (or 100 characters or more). It's very rare to go over the limit.
The error can be caused by having scanned imagines in your file which the system is unable to read/ scan in order to compare text or words.
This can be resolved by adding adding 20 words or more to the start or end of your work.
Please open the original file on your device
Add 25 words of text to either the beginning or end of your file
Save your amended file, and resubmit this version to Turnitin before the due date and time.