First off, you have a bigger problem.
The team that I've taken over doesn't currently do retrospectives or sprint reviews sadly.
You may have tagged the Question as scrum, but you're not doing Scrum. Before you try to fix any of the numerous issues that pop up as you try to implement your Scrumbut, you need to first try Scrum by-the-book. If you don't have experience with the baseline, then you won't be able to know how best to adapt Scrum for your organization. https://ronjeffries.com/xprog/articles/jatbaseball/
Why are you asking? For estimation, for reporting to management, or for planning?
For estimating? Ignore time entirely. Developers estimate in points, not time.
For planning? Look at past velocities and average them. If, over the past 5 (2-week) Sprints, you completed 84 Story Points, then the Team's velocity is 84 points over 10 weeks, or 8.4 points per week, or 1.2 points per day.
Do not try to plan for individuals. That's an anti-pattern. Plan for the Team. Likewise, you should be able to ignore time-off and the like; the averages will smooth that over.
For reporting? Do it in whatever format management expects. If points, give the points. If time, get the points, convert to time using the ratio determined above, then give the time.
if I ask for the team's availability during the sprint, and John indicates that he has PTO Thursday and Friday at the end of the sprint, and I know he is going to be out for a total of 16 hours. How does this information impact the total capacity if not calculating based on hours, and velocity has not been established?
There are three approaches I'd suggest suggesting to your Team and see which they prefer, or if they have a better idea.
A) Since planning is a fuzzy discipline in the first place, just ask the Team to take it into account when determining how many story points to take into the Sprint
B) Calculate how many man-hours are missing and multiply the rest by your historical velocity to determine how many story points to accept. If you have a Team of 3 and 1 person is missing 2 out of 5 days, then you're overall missing 2 out of 15, so multiply your velocity by 13/15.
C) Just don't worry about it. Overall your velocity will take PTO into account if you don't worry about it and average over enough data.