CAN ssh and connect though octopi.local, but cannot connect or ping IP
Since I am setting up octopi for muti-printers and I am following this video. https://www.youtube.com/watch?v=7Saa1HpLRoM
That means that I have to access octopi with an IP address.
I already try ssh with octopi.local. It can, and it shows the IP address. However, I cannot ping my computer on the pi or the other way around (ping the IP address as shown as ssh window)
I can ONLY connect my octopi using "octopi.local" and ping on my PC. However, it only shows IPv6 in my cmd when I ping it.
please help!
We don't really promote installing multiple printers per Pi; that's just a recipe for losing multiple print jobs all at once. Get one Raspberry Pi 3B for each printer to save yourself a lot of grief. In a case like this, I'd suggest issuing unique hostnames for each Pi.
If you can ssh into the Pi with just octopi.local and it connects you might then open another terminal on your same computer and run an nslookup octopi.local if this is Windows or a ping octopi.local to see if it returns an IP address or not. Let's say that it indicates 192.168.1.14.
Then back in the first successful ssh session, do an ifconfig and see if that IP address is the same to one of the network adapters (wlan0 or eth0). It's entirely possible that your network router only issued an IPv6 IP address to your Pi and not an IPv4 address.
When using an IPv6 you might have to surround it with square brackets if you're trying to use it.
ping octopi.local
ping -6 octopi.local # Windows
ping6 octopi.local # macOS and Linux, I'd guess
ping6 2601:648:8701:18c0:5e24:2fa2:b3cc:e96c # Works for me
ping 2601:648:8701:18c0:5e24:2fa2:b3cc:e96c # Would fail for me in this case
Thank you for replying.
So I should use one raspberry pi per printer, right?
That will be a little pricey for me as a student 
Is there any cheaper solution for 2-3 printers?
I have tried your suggestion. I can ping both "ping -6 octopi.local" and "ping -6 (IP addres)"
Here is my case:
My raspberry pi didn't connect to the internet (I cannot ping 8.8.8.8 but I can visit octopi.local on chrome.) though cable when I first set it up. (I connected with a switch to my LAN)
My solution was to let my pi connect both my wifi and switch. Then I could both let the raspberry pi to connect to the internet and visit on my PC.
Is that why I get into this problem? 
You say now that you're a student so I have to wonder if you're a student with three 3D printers or you're a student at school. The typical school has a variety of firewalls in place to prevent certain risky traffic; their firewall could be blocking the Pi's access to the Internet.
Yes, one 3B per 3D printer is the suggestion. A 3D printer is a manufacturing device moreso than just a inkjet printer or similar. Think of it as a fancy CNC machine in a factory.
Okay, so you've confirmed that the Pi has been issued an IPv6 address (which we knew from before).
So it sounds like the wifi side of your network has a feature called a DHCP service which didn't give you a default route (gateway) or it simply didn't want you to have one. Plugging into the eth0 network device with an Ethernet cable then got issued an IP address which did.
route
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
default 10.20.30.1 0.0.0.0 UG 202 0 0 eth0
default 10.20.30.1 0.0.0.0 UG 303 0 0 wlan0
10.20.30.0 0.0.0.0 255.255.255.0 U 202 0 0 eth0
10.20.30.0 0.0.0.0 255.255.255.0 U 303 0 0 wlan0
I've both plugged in an Ethernet cable and have connected a Pi4B via wifi to my 5Ghz zone nearby. I'm confident that my router is setup correctly because I am the awesomesauce.
In my case, I see two default entries listed under Destination: one for each network adapter. You should have at least one and whichever it is should work.
Note that the route -6 version of this has a lot more information and is decidedly harder to figure out. But again, you're looking for the line(s) which include UG for the Flags.