Q:

Trying to update macOS to Catalina 10.15.4 asks to restart again and again

When I open Software Update, I have:

An update is available for your Mac

macOS Catalina 10.15.4 Update

When I click on "Update Now", I have a window with:

To update, you must restart your Mac.

If I click restart, I effectively restart but when I try to update it asks
to restart again.
I have a MacBook Pro (Retina, 13-inch, Early 2015) and 12.29GB available.

A:

You need at least 20GB to install Catalina (macOS 10.15).  Updating this OS probably requires the same amount of space.
I’m guessing that your constant restarts are due to the installing not having enough free space so it’s swapping info from the disk to memory.
Try to free up as much disk space as you can, the more the better.  Once you get to 20GB, you should be ok.  This answer is based on my personal experience with updating many OS versions over the years.

