System uses 235gb of 251gb and its going on using rest of 4.4gb. If anyone knows how to fix this, I need urgent help.

You seem to be running out of storage, here:

How to free up storage space on your Mac

or

How to check the storage on your iPhone, iPad, and iPod touch

OmniDiskSweeper shows you the files on your drive, largest to smallest, and lets you quickly Trash or open them.

https://www.omnigroup.com/more/

Purging local backups

Please note that although this doesn't affect your remote backup from Time Machine, this will get rid of the redundancy (at least until the next Time Machine backup) that a local backup disk will provide. If you need such redundancy or are worried about the recovery of your data then you would be best served to let macOS determine when to purge these files.

Start Terminal from spotlight.

At the terminal type tmutil listlocalsnapshotdates.

Hit enter.

Here, you'll now see a list of all of the locally stored Time Machine backup snapshots stored on your disk.

Next you can remove the snapshots based on their date. I prefer to delete them one at at time. Once my "System" disk usage is at an acceptable level, I stop deleting but you can delete all of them if you want to reclaim all of the disk space.

Back at the terminal, type tmutil deletelocalsnapshots YYYY-MM-DD-HHMMSS , where will be one of the dates from your backup. This will be in the form of xxx-yy-zz-abcdef. Try to start with the oldest snapshot.

Hit enter.

Repeat for as many snapshot dates as required

http://www.thagomizer.com/blog/2018/03/27/cleaning-up-time-machine-local-snapshots.html

Erk sorry! I thought I saw this post in iOS...

Show us the disk in Disk Utility or “Storage” under About This Mac.

Like BD said; try a disk cleaning utility. I’ve used Clean My Mac X...

Make sure you are downloading a safe “cleaner”; as many fake ones may pop up every now and then.

Boot it via CMD-R and restore from a Time Machine backup. (Hope you have one...)

Or you can use single user mode (CMD-S) to boot it and delete files that way but using commands may be worthless...

Good luck

Go to settings>general>iPhone storage, screenshot the results including what takes up the most.