Unable to create subdomain under main domain in plesk control panel

Solution :

If you are unable to create subdomain for your domain the following may be the reason

[1] You have not allocated the subdomain resources in your service plan.

To resolve this issue you have to check and allocate the subdomain resources in your service plan

[1] Login to your plesk control panel.
[2] Click on service plan.
[3] Click on service plan name of that domain.
[4] Allocate the subdomain resources
[5] Click update and sync