I went to Sirius to engineer a couple power plants and was going to store the one on my taxi but it wouldn't let me. There was no option to store, only replace. So I traveled to another base and same thing. I have plenty of storage space and the option was given on other modules, just not power plants. Anyone else?
Powerplant is a required module; you can't have 0 powerplants installed, ergo the game will require you fit an E rated to replace. Hence the dialog to say 'replace' because it will replace with the
stock E rated module for that ship.
You will find the same will be true for all required modules. This includes Life Support, Distributor, Thrusters and Sensors; all, like powerplant, are required, all, like powerplant, will therefore say replace. Only optional modules, such as cargo racks, shields and so forth, with offer "store" (at least whilst space is available).