Olympia Archeological site and museum combo ticket

Here:

https://hhticket.gr/tap_b2c_new/english/tap.exe?PM=P1P&place=000000020

This is the official website.

"Skip the line" doesn't exist. It's a trick used by third-party sellers to charge you more.

Buy your €20 tickets on the official website and you'll automatically get a free "skip the line" access.

Thank you very much for your prompt response and link! I was trying to book for late September, yet unable so perhaps I'm too early?

This is because you want to book on September 27th or 28th. These are the European Heritage Days, entry is free to all archaeological sites this weekend.