“Prohibitory circle” at boot

This is the second time I’ve gotten this error graphic when I’ve rebooted my iMac Pro.
I’m installing the Catalina update at the moment, which will take care of Apple’s suggestion to reload macOS.
What else should I do?
How concerned should I be?

Edit: after powering off and on, it boots fine

Most likely a problem with the computer’s NVRAM/PRAM - first try resetting it. (See links below.) Secondarily it could be a corrupted boot drive, which can be repaired with Disk Utility.

Other possibilities include corrupted system files (which would require macOS reinstallation), or a disk drive starting to fail.

But for now just try resetting the NVRAM.

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I normally see this when someone has erased the HD/SSD in Mojave or Catalina incorrectly. Since your iMac Pro has 2 SSDs paired together… this might be the root cause for the prohibitory symbol to show up on boot. Have you erased the drive to start over fresh???

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Edited to add, I just power off and back on and it boots and runs fine.

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