What to do if I cannot complete my Masters dissertation tasks?
The unpleasant reality is that it is entirely possible that you may not pass your dissertation. In fact, given what you've written here, I would be surprised if you did as of right now.

Graduate level work is hard. I mean if research were easy everyone would be doing it right? And so some of the things Graduate students need to learn early in their career is time management, planning and asking for help as needed. In an ideal world your advisor would have caught your floundering much earlier in the process but academia, as life, isn't always ideal. A good rule of thumb is if, by the halfway point of your timeline(so 1 year into a 2 year Masters for example) you don't have a solid plan for accomplishing your goal that includes timelines, goalposts and a plan to catch up on additional material you need... then you have a problem.

By June, then, warning bells should have been going off in your head. I have some sympathy(everyone has to learn this stuff on their own at some point) but not too much. Part of graduate level Computer Science work, at least, means figuring out languages and implementations on your own. It sounds like you were looking for something a little more guided, more like undergraduate courses, but that's not how it works for Graduate level research. That you are a week out and just now casting about for a solution, that you, after running into the exact same problem when asking for help too late in June, just now found out that you could not implement improvements on a preexisting software suite... these are massively problematic. Ideally after the issues in June you should have chosen low hanging fruit for your last ditch effort of improving someone elses' suite. The first thing your lit review on that software suite should have contained is a list of potential pitfalls or limitations of the software and rough ideas on improvements.

Now that's all rather negative. I've written it because, from what you've written, you seem to not get what the fundamental problem has been for your research experience. All that being said there may still be solutions but it's going to be hard work.

It sounds like these implementations(different softwares) are focused on a particular field or topic. An option, a hard to do going to take overnights for the next week, option would be to do benchmarks and comparisons of currently available software. A study of the limitations and weaknesses(and strengths) of currently available software can be a viable topic. That it took you a month to set up the environment to run the software you were trying to improve means that this would potentially be very hard for you to do in a week. An option is to take the software you have now and run exhaustive benchmarks and analysis on that specific software.

I'm not suggesting this as a 'run out and do it now!' sort of thing. Honestly even the most on-the-ball student who has had no issues would be hard pressed to write a decent dissertation in a week. That's a student who has all of their data. But a passing dissertation is a possiblity. Your first step is to go talk to your advisor as soon as possible. Explain the current situation, explain a potential solution(exhaustive benchmarks on a particular software or comparative benchmarks between different software implementations) and, finally, explain what you are going to do to make sure that the day before the dissertation is due you won't have another emergency situation like you have right now.