The instructions proclaim that you need a screwdriver and that the rest is in the box. Simple. You may need a hammer; but no big drama. Oh, and a space larger than the one you have to lay out the pieces. A ten-minute job. The missing part is perhaps a sign, the wall attachment is “not part of the assembly” and you now need a level, drill, rawl plugs, and screws. Not the screws you have; longer ones, with flat tops. The instructions tell you to construct the item flat on the floor and stand it up; the ceiling is too low, and construction must be dismantled to stand it up. Getting the cupboard on the wall will require an assistant (not in the box), and reaching over to push the fittings home will need a ladder. I assume you don’t drill into cables or pipes, and that the plaster will not crumble.
The flat pack experience is very like software development or installation. It does not matter how many times you have built the kits, the excitement of discovery never goes away. Software development and deployment is like a two-dimensional flatpack viewed from the edge. Having someone who has the tools and experience, and understands what you want to achieve is good practice. Some turn to friends and family, others turn to @BeingResponsiv.