What if you can't publish in very high impact journal or top conference during your PhD?

The goal of a Ph.D. is to learn to be an effective scientific researcher. If you accomplish that during your Ph.D., then you will have been successful.

Moreover, early in one's career, high-rank publications don't actually correlate strongly with being an effective researcher. They're much more about happening to be in the right research group at the right time with the right guidance.

Now, if things line up for you and it happens that your Ph.D. also generates some high-rank publications during that time, it will make your next stage easier, but it is by no means necessary.

Remember:

• Most post-Ph.D. careers do not require high-rank publications. Highly focused fields, industry, teaching-centric colleges, and many other areas will care about the solidity of your work much more than its glamour.
• Interesting work often takes a long time to mature, and important work often shows up in high-rank venues only after it has been incubating in multiple publications in "bread and butter" venues over the course of years.
• If you do want a career that will be boosted by high-rank publications, one or more postdocs are an excellent period to obtain such. They also let you demonstrate that you are the important factor, and that it's not just that you happened to get lucky in your Ph.D. subject and advisor.

In short: stop worrying and instead just focus on doing something interesting and worthwhile with the work you're doing now.

Publications are important but not essential. Yes, peer-reviewed articles in higher ranking journals in your field matters in many fields. However, publications should not be seen as the only worthy outcome of your PhD. Many PhDs do not lead to publications but are still very worthwhile. Many fields do not focus on high impact journals, especially fields that are new with no established journal structures. Having a PhD in an up and coming field likely lead to high ranking articles later on. It may be harder to produce work in a parent field that may not be as supportive but persisting with the PhD to develop the field is still very worthwhile. A PhD will play an important role in establishing you in a moving field.

To see yourself as a failure without a high-impact article is setting yourself up for depression and ongoing anxiety. Fields such as the creative fields do not produce journal articles for example. Fields such as performing arts or fine arts where expression and setting fits better than traditional biblometric measures in other fields. Your environment and networks matter too. So if your institution does not have a track-record of producing research in high impact journals, then you have an unreasonable expectation. Your expectation and pressure to publish may not have the infrastructure during your time during your PhD. However, you have a longer term plan, you can set yourself up well for future collaboration that can lead to such publications.

Consider also whether your PhD is vocationally focused, ie. training you for a profession rather than just being research focused. Many PhDs are an entry way into various professions. For example, a PhD in data science would be highly valued in the current job market, even if you do not produce high impact articles. To expect a high-impact article from a PhD that is focused on professional training would unnecessarily discount the worthiness of your PhD.

TL;DR - your PhD is a means, not an end, so it's a fail only depending on what you aim to achieve by it. Focus on the research, the rest will follow.

if I can't publish in such top venues then my PhD is practically a fail.

This really depends on what you're trying to get out of it. If you are seeking to land a position in a top university then yes - you will most likely not be able to achieve this goal if you don't publish in top venues. That does not mean that your PhD will be a 'fail'. From a strict academic perspective - hiring committees/potential postdoc advisors look for your potential to make an impact. The main signal for that is publications. But how many and what kind varies widely. I have seen hiring committees prefer a candidate with one or two big publications over one with 4-5 mediocre ones (which to me makes sense, but to others it may not).

How will my PhD be valued if I cannot publish in top conferences or high impact factor journal?

The honest truth is that if you are not able to show capacity for publishing in good venues, it will not reflect well on you. However it is not the end of the world. You can still successfully graduate if you publish in other venues and your manuscripts are interesting. However, the burden of proof will be on you - you'll need to show how your work matters and why it is good. This is much harder to show than simply neatly stating your top-tier publications.