2 drives crashed on my TrueNAS core system, 4 drives in total on the machine. RAIDZ1. what should be the first step?
Rebuild the pool with replacement drives and restore from backup.
If what you say is accurate, that two drives really are dead, you lost[1][2] your data and it’s high time to check the status of your backups.
If you don’t have any backups, your Hail Mary would be that the drives aren’t dead, that something else is causing your issues, something that can be fixed.
If you describe your setup in detail others here will have a better chance to help you out.
Thoroughly describe your hardware, TrueNAS version, and so on.
The reason your data is lost is because RAIDZ1 only gives you redundancy to handle one drive failing. With two lost drives there isn’t enough data left to work with. ↩︎
Depending on the issue and how valuable your data is, a data recovery firm may be able to retrieve it for you. You will have to pay a premium to get their aid since recovery typically requires a clean room environment. ↩︎
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Explaining what you mean by “crashed”. Drives crash when they are moved, generally: I have yet to know a drive that can move, and crash with other drives perhaps, by itself. 
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And when you explain it, be as detailed as possible. It helps so these people helping do not need to make assumptions. Right now I assume you misundersand what has really failed as two drives failing at once is damn rare.
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Hmmm…
So far I see two issues - these two drives appear to be SMR drives which are not compatible with TrueNAS, also your system clock is some 11+ years behind the current time.
As already requested, please give a full description of your hardware, each system device, including the model number of each of your storage devices. Also tell us what TrueNAS version you are running.
People here will be able to give you considered advice based on that information. Right now there’s just not enough to go on.
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That will do it.
Who recommended to you the hardware for a TrueNAS machine? was it a Youtube video, those are out there. Did you read the recommended hardware requirements? And the question you might now want is, what is all your hardware? Only if you want some feedback as to if it is recommended stuff.
I know it isn’t what you wanted to hear however because you provided the needed information, at least you have the answer quickly.
Run zpool scrub -v NAS (assuming the pool name is “NAS”) and see what the result is. I suspect you will have a list of files that are corrupt, however it may not be many files, could be one of just a few. Post the output of the scrub if you need additional help.
If you haven’t already done so I recommend you backup your data.
You can use SMR drives in other NAS software, but the file system cannot be ZFS. So I’m saying all is not lost. You can still use those drives, just with something else.