The following issue has been observed to happen on Ubuntu computers (version 18.04 at least) sometimes when installing Sailfish X (version 188.8.131.52) to an Xperia XA2 device via a USB3 port. It is not clear where the culprit is, and if it would only happen with Ubuntu 18.04.
We present a workaround for it in this help article.
Soon after starting the script to flash Sailfish X to an Xperia XA2 the following can happen and appear at the computer terminal:
Searching device to flash..
Found 1 devices: CQ3000XMTM
Fastboot command: fastboot -s CQ3000XMTM
Flashing boot_a partition..
Sending 'boot_a' (17772 KB) FAILED (remote: 'unknown command')
Finished. Total time: 0.001s
The failure can hit when using a USB3 port on Ubuntu 18.04 computer (we are not yet aware of this in other environments).
How to avoid the problem
1) Check if there is a USB2 port  on your computer or on the USB hub you are using. Use the USB2 port, and then flashing should work.
2) If your USB hub has only USB3 ports, then connect the hub to PC with a USB2 cable, and XA2 to the hub with any USB cable ("USB C-type" in the XA2 end).
3) If there are only USB3 ports available, then this problem can be worked around in the following way on the PC:
- Force USB3 ports to USB2 mode temporarily (note that the 2nd command below is long - be sure to copy all of it)
lspci -nn | grep USB | cut -d '[' -f3 | cut -d ']' -f1 | xargs -I@ sudo setpci -H1 -d @ d0.l=0
- Install Sailfish X to your Xperia XA2 now.
In the end of a successful flashing session this script will write to the terminal:
Remove the USB cable and bootup the device by pressing powerkey.
- Restart your computer now to bring the USB3 ports to their normal mode. Alternatively, give the following reverting command:
lspci -nn | grep USB | cut -d '[' -f3 | cut -d ']' -f1 | xargs -I@ sudo setpci -H1 -d @ d0.l=1
 USB 3.0 Connectors are different from USB 2.0 connectors - they are usually (but not always) coloured blue on the inside in order to distinguish them from the 2.0 connectors. Check the specifications of your PC.