how to solve it with Large assemblies in SOLIDWORKS 2020

Try solidworks2020 by opening the file command to select the specified file, check the large assembly and then confirm

Hello many thanks for your reply, but really it didn't help , any other suggestion? maybe what are the best PC requirements for that

I have HP Z600 2XEON E5540 64GB ram Quadro K4000

Hello Nasir,
Although I really don't know what your problem is specifically, there is something that always happens: we make more and more complex models and the machine resources are running out of time.
In my country these resources are really very expensive, so we try some tricks when the models become big.
One of them consists in replacing the complete versions of certain parts and subassemblies with others that are simplified, provided that "at the scale in which we will look at the general assembly" this replacement is not noticeable.
Replacement parts can be generated in a number of ways to "save ram". The software provides a "large assemblies" mode with which it tries to do this, avoiding loading operations tree information and other information that is not significant and will not be used "at the scale in which we will look at the general assembly".
But this saving mechanism is not enough.
Other ways, increasingly thrifty and less precise, may consist of:
1) create versions of the pieces that do not contain details that will not be seen on a large scale anyway...
2) save parts and subassemblies as parts keeping only their exterior faces... (it's a "save as" option)
3) create equivalent geometries, very simplified, and place "decals" taken from the complete versions of those pieces...

You can see this last resource (and download the SolidWorks files) in my recent post:
Pillbug
You'll see that in some places the "Pillbug" set is seen up close and described internally, which is why I have to use its detailed version...
but in other cases (like when there are dozens of Pillbugs inside a spaceship) I use a simple cylinder to which I put "decals" that simulate the original piece reasonably well (especially when you see them from afar, on a large scale).

In the end, I hope the proposal works for you. If you want to share images of your project, I can help you by proposing some of these "resource saving actions".
Best regards!

PS: in the following images you have the full/detailed version of a Pillbug seen up close, and in the other, a distant view of a very large set of them. For this "distant view" I used a replacement part which, while imperfect, from such a large scale does its job of describing the system well.