If you have downloaded Brainstorm on Thursday, you caught a very fresh fresh bug that I introduced on Wednesday when reading epoched FieldTrip files:
https://github.com/brainstorm-tools/brainstorm3/commit/1499c1dd8ae8ae00b2bb28b31a20fba764fe1678
I've just fixed it, update Brainstorm (menu Update > Update Brainstorm to get it):
https://github.com/brainstorm-tools/brainstorm3/commit/cd71fd5e822ad2e26ed2fe1ba2e5275964640df6
If you're a new Brainstorm user, we'd recommend you go first through all the introduction tutorials in the section "Get started", using the example dataset, before trying to process your own files. You would save a lot of time on the long term, and would probably learn a lot of useful things:
https://neuroimage.usc.edu/brainstorm/Tutorials