Will I get refund on a cancellation of ticket of tatkal when it's already confirmed, but berth is not allowed yet?

this is explicitly what's mentionned in their website :

REFUNDS - No refund will be granted on cancellation of confirmed Tatkal tickets/duplicate tatkal ticket except in case of circumstances mentioned in para 2 of instructions contained in Commercial Circular no. 53 of 2006 issued vide letter no. 2006/TGII/ 20/P/Tatkal, dated 30.06.2006. However, full refund of fare and tatkal charges will be granted on the tickets booked under Tatkal scheme in the following circumstances :- If the train is delayed by more than 3 hours at the journey originating point of the passenger & not the boarding point if the passenger's journey originating point and boarding point are different. If the train is to run on a diverted route and passenger is not willing to travel. If the train is to run on diverted route and boarding station or the destination or both the stations are not on the diverted route. In case of non attachment of coach in which Tatkal accommodation has been earmarked and the passenger has not been provided accommodation in the same class. If the party has been accommodated in lower class and does not want to travel. In case the party travels in lower class, the passenger will be given refund of difference of fare and also the difference of Tatkal charges, if any.

The answer already provided gives you the rule. Let me give you a logical answer, tatkal scheme was introduced to address the needs of people who want to travel now, here (well almost), with just 1 day allowed as advance reservation period, Railways did not expect people to just book five tickets and cancel four later (which is an usual habit of many travellers) resulting in reduced ability to predict which tickets would get confirmed for others. So the contract here is you ask for a ticket I give it and then you cannot withdraw your offer after it has been accepted).

Now, if you are waitlisted in a tatkal ticket, then this whole principle is not applicable because your ticket unfortunately is not confirmed, the railways cannot allow you to cancel that and move on to a better way of travel. However if you chose to wait and your ticket gets confirmed, then your offer has been accepted and hence you cannot withdraw it. Allocation of a specific berth does not affect your request to give you a confirmed accomodation.