1. a problem or difficulty that stops you from achieving sth
the main stumbling block
A 'stumbling block' is an obstacle or problem that keeps you from reaching your goal. You could have 'a stumbling block in negotiations' or in a deal. You might 'hit a stumbling block' in a project or a life goal: "High rents are a stumbling block to savings for the millennial generation." You will hear this in social and professional contexts. Similar phrases include 'to hit a hurdle' and 'hit an obstacle'.