This is good sign (correct answer).
apt list uuu −− check if there already installed, if not installed:
sudo apt install uuu
Please hold Volume-up button, connect USB-A to USB-C cable to your Librem 5 and your laptop (still holding Vol+ button, battery is still out). When red LED light starts to blink carefully insert belonging battery (doesn’t matter if steady red LED light not there, just let us know which color you are having there, if any). Now you can move your finger away from Vol+ button 
.
This is important, please let us know if you are getting this output:
$ uuu -lsusb uuu (Universal Update Utility) for nxp imx chips -- lib1.4.193 Connected Known USB Devices Path Chip Pro Vid Pid BcdVersion ================================================== 2:1 MX8MQ SDP: 0x1FC9 0x012B 0x0001
EDIT: When, as needed and predicted, uuu -lsusb output indeed present (same as above), I would preferably avoid using Jumpdrive there (for those LED light reasons) and if you agree just follow this guide: External battery charger for Librem5. Of course that --skip-flash part of proposed guide needs to be removed. Actually you can leave it there for the test (first run) purpose, but for the real flash isn’t meant to be there. In short don’t use --skip-flash, when and if you decide to use this guide, now that your Librem 5 is ready to proceed … after turning-on every HKS (actually disabling power/function toward related chips/cards).
If some clarification needed please let me/us know.
EDIT II: Do not disconnect USB-A to USB-C cable right after flashing procedure DONE (whatever written there, on your laptop command line), please wait for more than several minutes until all install data transferred (relatively slowly) to your Librem 5 eMMC.
EDIT III: After flashing procedure finished (cable disconnected) please power-on your phone with the original power supply, by preferably using original USB-C to USB-C cable (especially if you are unsure that battery still holds enough capacity to finish related PureOS install).