Are you selecting the same interface that you are connecting to the network with?

First I select wlan0. which is what I am on. When I click my interference it shuts down my network manager and fern says: A problem occurred while setting up this mode of interference.

hear's what happens

You are using the wireless device that you're telling fern to use.... You need to use a separate device if you want to maintain networking as well as do the attacks. This is expected behavior that it would kill NM, since you're selecting the device you're using.

Why it's throwing the error is a bit more difficult to find out, and you'd likely need to run sudo fern-wifi-cracker from terminal to get actual output to debug it.

hello@kaliLinux:~$ sudo fern-wifi-cracker
[sudo] password for hello:
QStandardPaths: XDG_RUNTIME_DIR not set, defaulting to '/tmp/runtime-root'
libpng warning: iCCP: known incorrect sRGB profile
libpng warning: iCCP: known incorrect sRGB profile
libpng warning: iCCP: known incorrect sRGB profile
EXIT

This report has been filed against an old version of Kali. We will be closing this ticket due to inactivity.
Please could you see if you are able to replicate this issue with the latest version of Kali Linux (https://www.kali.org/get-kali/)?
If you are still facing the same problem, feel free to re-open the ticket. If you choose to do this, could you provide more information to the issue you are facing, and also give information about your setup?
For more information, please read: https://kali.training/topic/filing-a-good-bug-report/