For this trophy you need to have 1 million units in your inventory in one go, you cannot attain this in a cumulative manner through units earned the game.
The easiest way to earn this legitimately is to mine rarer materials such as gold and emeril and sell them at the trade stations or to the traders you encounter. You will also find good items in the big crates at save points on the planets you explore.
There is an item duplication glitch that can be used in the game to get this trophy more quickly but I haven't used it personally so cannot confirm if it still works or not.
Video by PS4Trophies: