a problem about macOS Mojave\

What version of macOS is currently running on this Mini? If you are running a newer version of macOS, then you will need to create a bootable macOS 10.14 Mojave USB installer instead since you cannot run an older installer on a newer OS. I highly recommend having a bootable macOS USB installer handy anyway since some people have trouble using Internet Recovery Mode to access the online macOS installers.....sometimes due to local network issues and other times issues with Apple's servers not allowing access to the older online installers.

Create a bootable installer for macOS - Apple Support

macOS Mojave 10.14.6 - Technical Specifications

General Requirements

OS X 10.8 or later

2GB of memory

12.5GB of available storage (OS X El Capitan 10.11.5 or later)*

Some features require an Apple ID; terms apply.

Some features require a compatible Internet service provider; fees may apply.

Mac Hardware Requirements

For details about your Mac model, click the Apple icon at the top left of your screen and choose About This Mac. These Mac models are compatible with macOS Mojave:

MacBook (Early 2015 or newer)

MacBook Air (Mid 2012 or newer)

MacBook Pro (Mid 2012 or newer)

Mac mini (Late 2012 or newer)

iMac (Late 2012 or newer)

iMac Pro (2017)

Mac Pro (Late 2013; Mid 2010 and Mid 2012 models with recommended Metal-capable graphics cards)

https://itunes.apple.com/us/app/macos-mojave/id1398502828?mt=12

Thanks to MartinR Terminal command…

softwareupdate --fetch-full-installer --full-installer-version 10.14.6

It will place a copy of the installer in your Applications folder and you can use that to create a bootable installer.

Double check the year model as per > Identify your Mac mini model - Apple Support

Then see > Update macOS on Mac - Apple Support

Note: for older macOS versions use Apple > System Preferences > Software Update.

BDAqua wrote:

Thanks to MartinR Terminal command…

softwareupdate --fetch-full-installer --full-installer-version 10.14.6

It will place a copy of the installer in your Applications folder and you can use that to create a bootable installer.

IIRC, that "softwareupdate" option only first appeared in macOS 10.15 Catalina and was undocumented at the time.

HWTech wrote:

IIRC, that "softwareupdate" option only first appeared in macOS 10.15 Catalina and was undocumented at the time.

macOS 11 Big Sur seems to be the oldest where 'softwareupdate --list-full-installers' was available.