Hello! First of all, congratulations on getting an interview! It is absolutely fine to send a follow-up email or call to ask about when they expect to have the interview time set. 

The most likely thing that is happening is that they are taking some time to gage different candidates' availabilities and put together a convenient schedule for the interviewers. As you can imagine, given many candidates in different time zones and interviewers with busy workdays, this matching process can be a little complex. However, there might be other things going on - it is quite uncommon (but not unheard of) for firms to send out invites only a few days prior to the interviews, but it certainly could happen. I actually had a similar experience as a candidate where the firm I was interviewing with was taking a very long time to set up my interview time - turns out they'd spelled my email address wrong, so I never got the email.

So I would say that it is never a bad idea to follow up, particularly if it will give you peace of mind. It certainly won't be looked upon negatively, and the worst thing you can expect to hear is that they just still need more time to confirm interview schedules. Just make sure you wait an appropriate amount of time before following up, and don't bombard them with calls or emails. I would say 2-3 business days after you sent your initial reply is an appropriate time window to reach out again.

Best of luck with the interview process!