You need to epoch these files before averaging them (import segments of them in your database).
I recommend you start by following the introduction tutorials (section “Get started” until #19) and the tutorial "EEG and Epilepsy), using the example datasets provided, before you try processing your own recordings. You will save a lot of time by getting a proper training instead of losing time trying things by yourself.
https://neuroimage.usc.edu/brainstorm/Tutorials