Does your User Account have Administrator privileges?
As long as the User Account you are using has Administrator privileges, I don't know why you would be experiencing trouble. SIP is not relevant. The installer app itself may be corrupted, which will require deleting it and downloading the High Sierra installer again. Open your Mac's App Store to do that, because an installer obtained from someone or somewhere else will not work.
I recommend the following, which requires a minimum of typing thereby minimizing the opportunity for error.
The following is the procedure I use and illustrates what you should see when creating a "bootable installer". Perhaps you will notice something different than what you have been experiencing.
To create a standalone installer on a USB flash drive or similar removable media, refer to the following instructions: Create a bootable installer for macOS - Apple Support.
Expanded instructions are as follows:
- Format the USB installer as Mac OS Extended (Journaled) and GUID Partition Map.
- Make sure its icon is present on your Desktop. Its name isn't important because createinstallmedia will change it.
- Also, the Install macOS High Sierra app must be present in your Mac's Applications folder, otherwise the pathname shown below won't be correct.
- Make the Finder's Applications window small enough so that you can see it, the Terminal window, and the USB drive icon, all at the same time.
Select the following line by dragging your cursor from just prior to the word sudo to just past the word volume. Don't triple-click the entire line and don't drag your cursor below the line:
sudo /Applications/Install\ macOS\ High\ Sierra.app/Contents/Resources/createinstallmedia --volume
Confirm that nothing is highlighted other than that line. Nothing before sudo and nothing after volume.
Then, Edit > Copy.
Go to the Terminal window, paste that line into the Terminal window, and type a space character. Stop right there; do not press Return.
Drag the SD card or USB flash drive icon from your Desktop to the Terminal window, and stop again.
Then, type (or copy/paste) the following into Terminal:
--applicationpath
... followed by another space character.
Then, drag the Install macOS High Sierra icon from your Applications folder into the Terminal window.
Finally, you can press Return.
This is an example of what should happen. The name of the USB drive is "untitled" in the following:
MacBookAir7:/ john$ sudo /Applications/Install\ macOS\ High\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/untitled --applicationpath /Applications/Install\ macOS\ High\ Sierra.app
Password: (type your Admin password - it will not be echoed, not even with •••• characters. Then press Return)
Ready to start.
To continue we need to erase the disk at /Volumes/untitled.
If you wish to continue type (Y) then press return: Y
Erasing Disk: 0%... 10%... 20%... 30%...100%...
Copying installer files to disk...
Copy complete.
Making disk bootable...
Copying boot files...
Copy complete.
Done.
MacBookAir7:/john$
When it's finished the name of your USB or SD card installer will be Install macOS High Sierra and it will be selectable as a bootable device in Startup Manager: How to choose a startup disk on your Mac - Apple Support.
