One reason may be related to the storage capacity of the USB stick, as 8GB at this time may be insufficient, most likely due to the need to use a swap file (virtual memory) for which there is not enough space available; it could also be a limitation due to the limited read/write speeds of the USB stick.

If you are unable to boot in Guest mode from the USB stick, you can try a faster storage medium with a capacity of 16 or even 64GB because there are no high-performance low-capacity devices on the market anymore. However, we can see that the booting problem persists even though sufficient hardware resources are available (including processing power and RAM capacity/speed), in which case we will want to install Chrome OS Flex on the internal storage. By default, this will erase all content (formatting), in conclusion, it is not recommended to use it on a device from which data recovery is required.

Unfortunately, at least for the 2024/2025 version, testing ChromeOS Flex on virtual machines is not immediately accessible, as it is necessary to use/clone an existing ChromeOS Flex installation because usually the virtual machine cannot boot from the external storage with the Chrome OS Flex image applied; also the image file is in BIN format, not ISO and cannot be used directly for installation on the virtual machine.