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Hi Dhanush,
Usually in all numerical calculation methods, there are relations between different global parameters. For example the temporal window and the spectral window, the number of samples and the step size in time domain and frequency domain.. and so on. Without these relations, numerical errors and accuracy are huge. sometimes the results do not make sense.
It is very important to see the above point.
let use come back to your question. the sequence length is related to the Bit rate and the Time window, as written in the manual of the global parameters. So when you changes the sequence length to this huge number (2097152, which is ok for me) either the Time window or the Bit rate changed accordingly based on which one you forced their value to be based on your choice (from the selection in Simulation window). so for example, if you select ” set bit rate” in the “Simulation window” and left it to its default value “10e+009”, then when you change the sequence length to (2097152) you will find the “Time window” becomes “0.0002097152 s” which is very wide window so consequently the frequency window will be very very narrow!!
All these changes will affect the numerical calculations, this might be the reason of the program hanging.

I wanted to describe to you the above points to clarify the issue. Hope it is clear now.

I suggest you to adjust the Bit rate and the time window values in such a way make the calculation reasonable with this huge sequence length.
(BTW, I noticed that your sequence number is 2^21 which is good to have it in such a way that can be represented as 2 to the power integer! )

please let me know if the above discussion is useful to go further with your project or not.

Best regards,
Mohamed Abouseif