I have a RPI 3B with the latest Buster on it. Attached is a Strompi 3 with accu pack.
The power comes form an official Raspberry Pi supply.
I have followed the steps in the book and still it is impossible to get in touch with the console via minicom. The strompi is not in flash mode.
First, remove all voltage supplies and remove the CAP jumper for a few seconds. Then put the CAP jumper back on (ON position). Now start your system again and test if it works. If it doesn’t work I would recommend you to try one of our prepared images (lite / full) and test if it works with a prepared image.