Problem with 80TTB in Efiling

I trying to fill in my details in the Tax Utility "ITDe-Filing-2024 Setup 1.1.0".

Unfortunately, I am stuck up at the 80TTB.

Initially, the Utility was showing a greyed out figure of Rs. 12,500/- ( I do not know how this value is obtained by the Income Tax). But when I select "I don't agree", I am shown an Instruction as per the screen shot attached, herewith. 

Firstly, I could not find a provision to get the 80TTB (this is no where available in the Utility).

Secondly, whatever value I enter is ignored in the deduction by the Utility, for tax computation.

I am puzzled - kindly help!