I've just met this frustration, tried three sd cards etc but eventually got to login to the web interface without effort so here is my track should anyone come here again. (In reality I only needed to use one of those sd cards).
Get a really useful tool that is something like Network Utility (built-in app on Mac). Firstly you'll use this to find your RPi IP address to enter into the browser. Secondly you can do a Port scan on that IP to see if port 80 is open. The port 80 web interface is inactive during the 20 minute wait. Luckily I found this page to read about about the 20 minutes wait!
SO IN SUMMARY:
Use etcher to put the RPi OMV image on the micro-SD ... etc.
Boot and watch the screen. Don't try to login to the web console yet*. Wait 20 minutes or watch the network activity light flashing meanwhile. The system will do some update and reboot and then you can enter the web console.
*BTW When the text stops after an initial few minutes you CAN press spacebar for the ip address and a login prompt. You CAN login with admin and openmediavault but this did nothing useful for me. Best wait the 20 minutes.