How to export Sales Navigator leads to Excel has a short answer from LinkedIn: you can't. LinkedIn's help page on exporting account and lead information says, "LinkedIn currently doesn't offer the option to export account and lead information from Sales Navigator into a CSV or XLS file." No plan has a download for it, Advanced Plus included.
What looks like an export but isn't
A few things in Sales Navigator sound like an export at first. None puts your leads in a spreadsheet:
Saving leads to a lead list keeps them inside Sales Navigator.
Embedded Profiles shows Sales Navigator details on a record inside your CRM. It copies nothing out.
CRM Sync, on Advanced Plus, creates contacts in your CRM, and in some CRMs leads. Any spreadsheet then comes from your CRM's own export, under its rules and your admin's, not from LinkedIn.
The helper above lists each one with the plans and CRMs LinkedIn names for it.
How to read the helper
It opens on LinkedIn's sentence, quoted word for word, with a link to the page and the day we read it, so you can check the wording yourself. Pick any plan and the file export stays unavailable. Picking a CRM does not change it either.
The one thing from a search you can put in a spreadsheet is the search itself. Paste a Sales Navigator search link into the reader and it lists each filter with its values in plain words, ready to copy. The link is read in your browser tab and never sent anywhere.
What to do next
Ask what the file was for. If it was a CRM import and you are on Advanced Plus, CRM Sync already creates the contacts without a file.
If you need a spreadsheet of people with work emails, start the list somewhere that allows a download. In Oneprofile, a people search filters by title, seniority, and location, finds work emails as you save, and downloads the table as an Excel file. Getting an email from a LinkedIn profile covers the case of one person.