What do you do if there is not enough disk space for an update which needs 16.72 gb

We cannot trust the Storage report as to where the usage really is,4 suggestions…

Have you emptied the trash lately?

Look for iOS backups…

/Users/[USERNAME]/Library/Application Support/MobileSync/Backup

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

You would click on the startup drive icon (Macintosh HD) on your Desktop and press option+command+i to reveal a Get Info panel (that does not hide under your open browser window). You can trust the Available storage reporting there.

Your profile shows macOS Sierra, and you have posted in a macOS High Sierra community. If you were upgrading from Sierra to for instance, Big Sur, you would need 45 GB of Available storage, and from High Sierra or later, 35.5 GB of available storage.

You would look at your home directory and in particular, how much storage is consumed by iTunes music, video, or iOS application updates, large email attachments, and large Photo libraries and downloads.