DFU error with NRF Toolbox

no direct experience with the bare module but like the Puck it needs to be in boot loader mode before the DFU is available. You don’t say if you have others that you can “flash”. The docs page on here may help if this is your only module. In the absence of a button you need to make a wired connection. I don’t know what that is but it is in the docs, I recall reading it, when I was considering the module.