Jordan
It used to be usual for a new claimant for ESA to be invited to a Work Focussed Interview during the assessment phase, the practice seemed to have dropped off in the last few years, but we have seen it happening much more this year.
If it is WFI (which I think it probably is), then your attendance is required, only claimants in the Support Group are excused from these, failing to attend could and most probably will result in your ESA being sanctioned.
If it is not a WFI but rather comes under the heading of Work Related Activity, then you need to confirm whether your attendance is mandated or not, it is unlikely that it will be optional but it could be, a failure to attend a mandated activity will result in your ESA being sanctioned.
Call the JC+ office, there should be a phone number on the letter to find out what is going on.
Gordon
Nothing on this board constitutes legal advice - always consult a professional about specific problems