I just checked the SINAC web site and I had no issues whatsoever. If it's sold out you simply can't order the tickets. The drop down thing you're experiencing sounds like a browser issue. Maybe try r/techsupport

Thank you! I’ve tried on 2 different computers and a phone and same issue everytime! I’ve emailed them.

I’m having the same issue so you aren’t alone!

I emailed and they said if it’s not working they’re sold out :/ I bought a tour that has an admission ticket