Can be poor / inadequate USB connection, perhaps look at the whole USB chain. It has been reported that certain USB will not carry fastboot commands or complex fastboot commands. You will need a good quality (full capacity) data cable, so not a thin, skinny cable.
Avoid USB hubs including any USB hub that might be hidden in the PC / laptop.
Avoid a USB-C to USB-C cable as it may be provisioned through a hidden hub.
For predictably good results check (if necessary with a manual) that you are connecting to a USB-2 port soldered to the motherboard.
Otherwise using the manual command line method is probably easier than trying to debug eosinstaller. For tetris the install is by script so not so hard. Install /e/OS on a Nothing Nothing CMF Phone 1 - “tetris”
It is possible that your Chrome (variant) browser does not fully support WebUSB. Brave has been suggested problematic in this respect.
There is also [HOWTO] a Safe Guide to install "IMG-e-builds" even if the /e/ web installer fails