I just tried unsuccessfully to download macOS Mojave. Is it still available?

Still available:

https://support.apple.com/en-gb/HT211683

LabMaster Said:

"I just tried unsuccessfully to download macOS Mojave. Is it still available?"

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USB Bootable Installer:

Yes. It is still available. If it isn't working, try using a USB Bootable Installer instead. Go Here: How to Create a Bootable Installer for macOS - Apple Support

I just tried unsuccessfully to download macOS Mojave.

Use Safari for any downloads. FireFox has issues with Mac system files lately.

If, when running the installer, you get a message about a damaged file, don't panic. It is simply an ill-worded advisory that the current installer has found an outdated version of itself already on the computer. Open your Applications folder, find the old installer and trash it. Then you may proceed using the current installer.