Three weeks in I would expect you to be teaching some lessons from your class teacher's planning, maybe adapting the planning to change an activity or two, keeping in line with the learning objectives.
I might ask you to plan a section of a lesson, giving you a very clear learning outcome for that section.
Noone should be planning to the title 'Light'.
We have had the problem of (unmoveable because of othet timetabling) PPA being on uni day for some trainees. For some we suggest working on other aspects first (eg. behaviour management, teaching to the teacher's planning, marking to the learning objective etc) first, then becoming an apprentice planner when the placement full weeks start. For some other trainees, the class teacher has found another time in the week to discuss planning with the trainee.
It may be that, because you were a capable TA, your class teacher and mentor(s) assume you actually know it all, and the training year is largely a formality for you.
Speak up and tell them what you need support with at your next mentor meeting, or before.
Ask for the long or medium term plan for science and/ or which objectives should be covred over what time period. Ask what the children's previous experience of the 'light' topic has been and what their recent experience of science has been eg. Did they do any during lockdown?
These are all essential questions to ask before you - or any experienced teacher- could plan effectively for this class.
Unconscious incompetence is 100% normal at this stage - and a good sign. Don't let it cripple you, but ask for help.