The brand new SPEKTRUM AR7200BX / AR7210BX / AR7300BX only shows a red flashing LED or even nothing. What's wrong with it?

Carefully check whether all cables are plugged in correctly and all the plugs are connected properly at the correct position. Especially in the vertical direction check that not just 2 pins sit on the header. Also make sure that the polarity is correct on all connections. Remove all servo connections to make sure that there is no defective servo pulling too much power.