"Reset Security Token" Tab missing in Salesforce Org?

Hi Suraj
There are few possible scenarios where the user cannot request a security token.

1. Verify if IP Restrictions are added to the Login IP Ranges
If there are any IP range values defined then Reset My Security Token option will not be available.
(E.g. Start IP: 192.162.0.0 _ End IP: 192.162.0.255)

In order for the option Reset My Security Token to appear you will need to first either remove the Login IP Ranges or change the user to a profile that does not have any Login IP Ranges listed.

Salesforce classic UI

If the Salesforce edition is Unlimited, Performance or Enterprise, System Administrators can verify if there are IP values in the Login IP ranges by selecting: Setup | Manage Users | Users | Profile| Profile name | Login IP Ranges.
If the Salesforce edition is Professional, Group or Contact Manager, IP Restrictions are located in the Session Settings, instead, system administrators can verify this by going to: Setup | Security Controls | Session Settings.
New Lightning UI

If the Salesforce edition is Unlimited, Performance or Enterprise, System Administrators can verify if there are IP values in the Login IP ranges by clicking on the top right Gear | Setup Home | Users | Profiles | click on the Profile name | Login IP Ranges.
If the Salesforce edition is Professional, Group or Contact Manager, IP Restrictions are located in the Session Settings, instead, system administrators can verify this by clicking on the top-right Gear | Setup Home | Security | Session Settings.
2. Verify if the Network Access selection is missing from Security Controls setting
Follow these steps:

Go to Setup
On the left pane of the "Quick Find" box, enter the words “Network Access"
If you have a non-administration profile, contact your internal System Administrator to perform the steps for you because you may not be able to see the Network. A System Administrator can see the Network Access option.
Note: If you don't see the "Network Access" link above, the system administrator can log a case with Support to request the following features enabled for the Organization:
- Login Challenge Enabled
- Login with API Token

3. Verify if user's Profile is corrupted
If none of the previously mentioned scenarios are true in your case, try editing the profile of the user that's missing the security token and then saving it without making any modifications. This will reset the profile, and might result in the token option being available. If the information provided is unable to help resolve the issue please log a case with support for assistance.

Hope this may help you:

thanks