It's a psn issue. Because your secondary account probably didn't buy elite (just your primary assumably), it doesn't have a license for using the dlc. Previously this was how ps4 cmdrs could have multiple alts for free whereas PC had to purchase alts 1 at a time. At this point nothing you can do about it but buy it again for the alt psn account or forget about it.
Apparently some players say that playstation reported 3 times of the account policy change to remove the privileges to use dlc etc, frontier I do not know if he said something I did not know anything about that, but in the game the horizons appears as installed and it cannot be reinstalled, unless the game is reinstalled and asks to reinstall horizons, which I do not know.This started happening to me from yesterday. I only have the one account, (PS4), and it is very annoying, as i rely on planetary landings to provide materials for limpet synthesis and AFMU.
It's a psn issue. Because your secondary account probably didn't buy elite (just your primary assumably), it doesn't have a license for using the dlc. Previously this was how ps4 cmdrs could have multiple alts for free whereas PC had to purchase alts 1 at a time. At this point nothing you can do about it but buy it again for the alt psn account or forget about it.