Can't hide nav items in portfolio subpages

Site URL: https://gmengenheiros.com.br/projetos/sap-labs-en

I've been translanting an existing site to english and I've been having trouble targeting subpages in a portfolio (#item-...) Don't know jack s*** about css coding and have been bumping my head for 1 week now haha. Here's the code I used site-wide, and it worked: 

<style>
#header .header-nav-item:nth-child(6) {display:none}

#header .header-nav-item:nth-child(7) {display:none}

#header .header-nav-item:nth-child(8) {display:none}

#header .header-nav-item:nth-child(9) {display:none}
  
  #header .header-nav-item:nth-child(10) {display:none}
</style>

So I thought this would work, when targeting an specific subpage, but it does not:

<style>
#item-5fd8ad80acce81371bfc75c8 {
#header .header-nav-item:nth-child(9)      {display:none}
 !important }
  </style>

I'd really appreciate if someone could help, thanks in advance!