The comment field is not displaying. What should I do?

This issue usually occurs because the Comments option is disabled in the particular user’s profile permissions.
Note:  Only the Admin user can access the profile permissions settings.  
Follow these steps to enable the Comments box for standard users in Vtiger CRM:

1. Log in to the CRM     
2. Click User Menu.
3. Select Settings. The Settings page opens. 
4. Select Profiles. The Profile page opens.
5. Select the Profile assigned to whom you want to enable the comment option. 
6. Go to Other modules > Comments and enable the required permissions ( Add, View, Delete or Edit Comments).
7. Click Save.