Being a singer and singing teacher (which immediately arouses suspicion in folk person) I come across many people who have been told they cannot sing

Reasons are

1 Not using enough breath to create tone - voice needs support from diaphragm as well to sustain a voice over a period of time

2 Not breathing enough between words or phrases

3 Not opening throat properly

4 Not singing in right part of voice or key

5 Clenching or bracing throat and shoulders to create unwanted tension

6 Vocal damage due to drink - nicotine or other substances

7 Not exercising the voice enough - ideally voice should be worked every day

8 Hearing problems

A lot can be done with proper training and by recording the voice and finding what works to build up muscle memory