Booted to Recovery Mode your boot volume is a mounted base macOS image residing in a subfolder of the recovery partition. It doesn't contain a /Users folder and if so it would be the wrong one. So cd to your main volume instead.
To check and modify your standard boot volume you have to get its volume name and device name first. Open Terminal in the menubar > Utilities:
mount #get volume names and disks
diskutil list #get disk identifiers of internal disk and CoreStorage: usually disk0 and disk2 for CoreStorage Logical Volume
diskutil verifyDisk /dev/disk0
diskutil verifyVolume /dev/disk2 #CoreStorage
diskutil verifyVolume /dev/disk0s2 #no CoreStorage
Try to repair any error by replacing the suffix verify with repair.
Now cd to the volume (example name here: "Macintosh HD"):
cd /Volumes/Macintosh\ HD
Now cd to the Users folder and your user folder:
cd username #with username: usually your short username
Remove dispensable user files. Other places to remove files from are /Volumes/Macintosh\ HD/private/tmp, /Volumes/Macintosh\ HD/cores or /Volumes/Macintosh\ HD/private/var/vm. Replace "Macintosh\ HD" with the name of your main volume here.
Addition: if your disk is not automatically mounted in recovery mode (e.g., because it is encrypted), you can mount and decrypt it by quitting the Terminal utility and going to the Disk Utility. Select your drive in the left column and mount it using the mount button (in the window at top right). The system will ask for an administrator password to decrypt. After mounting you can quit the Disk Utility and go back to the Terminal again. The disk will stay mounted.