You didn't tell us which version of the OS you're using.
The last version of iPhoto (9.6.1) was released in March 2015.
The d5600 is relatively new.
I'm guessing that the RAW files generated by the d5600 are "too new" for iPhoto to "digest" any more, particularly if you're using an older version of the Mac OS.
There may be "workarounds".
You realize that there's a dedicated sub-forum here for digital photography?
Also, you might visit the forums at dpreview.com as well.