integrity constraint violation: 1452 cannot add or update a child row: a foreign key constraint fails
It just simply means that the value for column project_id on table comments you are inserting doesn't exist on table projects. Bear in mind that the values of column project_id on table comments is dependent on the values of ID on table Projects.
The value 50dc845a-83e4-4db3-8705-5432ae7aaee3 you are inserting for column project_id does not exist on table projects.