azk: unknown version :mountain_lion

[NigelThorne]

1. What are you trying do accomplish?

Install on Mac OS Catalina (v 10.15.3)

2. Steps to reproduce the described behaviour

Run installation script.

3. Output

==> Tapping azukiapp/azk
Cloning into '/usr/local/Homebrew/Library/Taps/azukiapp/homebrew-azk'...
remote: Enumerating objects: 352, done.
remote: Total 352 (delta 0), reused 0 (delta 0), pack-reused 352
Receiving objects: 100% (352/352), 38.70 KiB | 202.00 KiB/s, done.
Resolving deltas: 100% (169/169), done.
Error: Invalid formula: /usr/local/Homebrew/Library/Taps/azukiapp/homebrew-azk/Formula/azk-rc.rb
azk-rc: unknown version :mountain_lion
Error: Invalid formula: /usr/local/Homebrew/Library/Taps/azukiapp/homebrew-azk/Formula/azk.rb
azk: unknown version :mountain_lion
Error: Cannot tap azukiapp/azk: invalid syntax in tap!
[azk] Installing azk..........................................[ FAIL ]

[azk] [ERROR] Failed to install azk. Try again later.
[azk] [ERROR] Failed to install azk.

4. Further info

Which VirtualBox version? (If running Mac OS X) - 6.1