I forgot to add preprint submission (BioRxiv) before the actual journal submission, what can I do now?
You are being very careful, though inexperience, and this speaks to your good will. It is no big deal. Just submit the preprint and notify the journal, stating that preprint submission was done after journal submission. Most journals have a "contact" link in the "action menu" for your submission that will contact directly the person managing your submission. If you cannot find such a link, look for a "managing editor" rather than "editor in chief," and make sure to include your submission number in the communication.

I would also let your PI know. There should be no reason for them getting angry at you, and it will make you look good that you own your mistake and try to rectify promptly.