At Act IV, after returning from the Tyrell's office with his DNA pattern, I can neither give it to the Twins, nor retrieve the evidence for Guzza from the console when the twin is not in the lab.

The only way I can get the folder for Guzza is to shoot the Twins repeatedly but I was still unable to call Guzza, as described in the walkthrough, in order to proceed. Thus, I had been walking around the sewage across various locations but still can't progress the plot.

Any idea how I can continue the story?

Extract from Walkthrough
//
-------------------------
DERMO DESIGN - TWINS' LAB
-------------------------
Give them Tyrell's DNA pattern information and they'll give McCoy a folder
containing evidence that Guzza is a dirty cop.

Go upstairs to their office.

NOTES
-----
If McCoy retires the Twins, he'll have to access the console on the right to
pick up the folder.

When the Twins are not around, McCoy can pick up their DNA pattern
information on the console next to their chair. He won't be able to pick it
up if he kill the Twins, which causes them to overrun the computer from
their heads hitting the keyboard. This can be avoided if McCoy shoots them
until the crosshairs turns grey.

-----------------------------
DERMO DESIGN - TWINS'S OFFICE
-----------------------------
Call Guzza through the old vidphone on the right. McCoy will tell him to
meet him at the central sewers alone.

Go back down to the lab.
//

Hi, do you have a save file available for this situation? I could have a look into it and see what's the issue, and if it is indeed some bug.

Sorry for the delay. I managed to obtain the folder for Guzza without killing the twin but yet to be able to make the call. Please see the linked save files for further analysis.

[Link Removed]

I've had a look in your save games. There is a way to progress the story. You can continue from the latest one, named "A".

So, apparently a few clues missed in the previous Acts and a few decisions you took made it a bit harder for McCoy to reach to certain suspicions about a certain someone, so that he could talk trade with the twins. Luckily you can still get one clue that will open up the option to talk more with the twins.

This clue is at Izo's underground stash and is related to the weapons. There's a way to get to Izo's basement from the sewers: https://imgur.com/AByHAoW

After you get the new weapons clue, go back and talk to the twins.

Let me know if you need more clues or if you'd like me to tell you exactly what you have to do.

Thank you so much for your help and I can finally beat the game. What a throwback to the original Blade Runner.