Dear Esperanza Llorens Rodriguez,
Good day!!
Apologies for the inconvenience caused and please don’t worry, let’s work together on your concern and move towards a resolution path.
I understand your concern and to my knowledge, I would like to convey that Site Collection Administration in the Site Settings of the Site will be available only to the Site Collection admins of the Site.
To get the Site Collection Admin role on the SharePoint Online site, SharePoint Online admin of your organization will be able to help you to provide the Site Collection admin on the site via below steps:
Open the SharePoint Online admin center> Active Sites> Click on the site name where the Site Collection Admin need to be added> Membership tab> Site Admins> + Add Site Admins> Give a name and click on Add.
Note: The above steps can be performed either by the SharePoint Online admin or IT admin of your organization.
If there is any misunderstanding or confusion, we apologize and request you to share the more detailed information which will help us to better understand and guide you further.
However, if the information provided by me on your concern has helped to clear your doubts, it would be very helpful if you could mark the reply that helped you by voting the options under "Was this reply helpful?” as you can see here:

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Thanks a lot,
Sophia