It will be missing this component:
https://github.com/arduino/ArduinoCore-API
You can see how it is added during the release creation here:
https://github.com/arduino/ArduinoCore-samd/blob/1.8.12/.github/workflows/release.yaml#L16-L37
Installing directly from GitHub is useful for beta testing non-release versions. For manual installation of release versions, you will probably find it more reliable and simple to download the release archives from the URLs listed in the package index:
https://downloads.arduino.cc/packages/package_index.json
You can see the entry for the arduino:samd@1.8.12 release:
{
"name": "Arduino SAMD Boards (32-bits ARM Cortex-M0+)",
"architecture": "samd",
"version": "1.8.12",
"category": "Arduino",
"help": {
"online": "http://www.arduino.cc/en/Reference/HomePage"
},
"url": "http://downloads.arduino.cc/cores/core-ArduinoCore-samd-1.8.12.tar.bz2",
"archiveFileName": "core-ArduinoCore-samd-1.8.12.tar.bz2",
"checksum": "SHA-256:13c44be50367bb0460cd8ff24a43c3bff1b9e9b209c5ce25595a9d99d080a3a1",
"size": "3100138",
"boards": [
{ "name": "Arduino MKR WiFi 1010" },
{ "name": "Arduino Zero" },
{ "name": "Arduino MKR1000" },
{ "name": "Arduino MKRZERO" },
{ "name": "Arduino MKR FOX 1200" },
{ "name": "Arduino MKR WAN 1300" },
{ "name": "Arduino MKR WAN 1310" },
{ "name": "Arduino MKR GSM 1400" },
{ "name": "Arduino MKR NB 1500" },
{ "name": "Arduino MKR Vidor 4000" },
{ "name": "Arduino Nano 33 IoT" },
{ "name": "Arduino M0 Pro" },
{ "name": "Arduino M0" },
{ "name": "Arduino Tian" },
{ "name": "Adafruit Circuit Playground Express" }
],
"toolsDependencies": [
{
"packager": "arduino",
"name": "arm-none-eabi-gcc",
"version": "7-2017q4"
},
{
"packager": "arduino",
"name": "bossac",
"version": "1.7.0-arduino3"
},
{
"packager": "arduino",
"name": "openocd",
"version": "0.10.0-arduino7"
},
{
"packager": "arduino",
"name": "CMSIS",
"version": "4.5.0"
},
{
"packager": "arduino",
"name": "CMSIS-Atmel",
"version": "1.2.0"
},
{
"packager": "arduino",
"name": "arduinoOTA",
"version": "1.2.1"
}
]
}
So the release archive is available for download here:
http://downloads.arduino.cc/cores/core-ArduinoCore-samd-1.8.12.tar.bz2"
But notice the platform has all these tools dependencies:
"toolsDependencies": [
{
"packager": "arduino",
"name": "arm-none-eabi-gcc",
"version": "7-2017q4"
},
{
"packager": "arduino",
"name": "bossac",
"version": "1.7.0-arduino3"
},
{
"packager": "arduino",
"name": "openocd",
"version": "0.10.0-arduino7"
},
{
"packager": "arduino",
"name": "CMSIS",
"version": "4.5.0"
},
{
"packager": "arduino",
"name": "CMSIS-Atmel",
"version": "1.2.0"
},
{
"packager": "arduino",
"name": "arduinoOTA",
"version": "1.2.1"
}
]
So installing the platform release archive on its own will not be sufficient unless you happen to have the necessary tools already installed on your system.
The tool archive download are also listed in the package index. For example, here is the entry for the arduino:arm-none-eabi-gcc@7-2017q4 dependency:
{
"name": "arm-none-eabi-gcc",
"version": "7-2017q4",
"systems": [
{
"host": "arm-linux-gnueabihf",
"url": "http://downloads.arduino.cc/tools/gcc-arm-none-eabi-7-2019-q4-major-linuxarm.tar.bz2",
"archiveFileName": "gcc-arm-none-eabi-7-2019-q4-major-linuxarm.tar.bz2",
"checksum": "SHA-256:34180943d95f759c66444a40b032f7dd9159a562670fc334f049567de140c51b",
"size": "96613739"
},
{
"host": "aarch64-linux-gnu",
"url": "http://downloads.arduino.cc/tools/gcc-arm-none-eabi-7-2018-q2-update-linuxarm64.tar.bz2",
"archiveFileName": "gcc-arm-none-eabi-7-2018-q2-update-linuxarm64.tar.bz2",
"checksum": "SHA-256:6fb5752fb4d11012bd0a1ceb93a19d0641ff7cf29d289b3e6b86b99768e66f76",
"size": "99558726"
},
{
"host": "i686-mingw32",
"url": "http://downloads.arduino.cc/tools/gcc-arm-none-eabi-7-2017-q4-major-win32-arduino1.zip",
"archiveFileName": "gcc-arm-none-eabi-7-2017-q4-major-win32-arduino1.zip",
"checksum": "SHA-256:96dd0091856f4d2eb21046eba571321feecf7d50b9c156f708b2a8b683903382",
"size": "131761924"
},
{
"host": "x86_64-apple-darwin",
"url": "http://downloads.arduino.cc/tools/gcc-arm-none-eabi-7-2017-q4-major-mac.tar.bz2",
"archiveFileName": "gcc-arm-none-eabi-7-2017-q4-major-mac.tar.bz2",
"checksum": "SHA-256:89b776c7cf0591c810b5b60067e4dc113b5b71bc50084a536e71b894a97fdccb",
"size": "104550003"
},
{
"host": "x86_64-pc-linux-gnu",
"url": "http://downloads.arduino.cc/tools/gcc-arm-none-eabi-7-2017-q4-major-linux64.tar.bz2",
"archiveFileName": "gcc-arm-none-eabi-7-2017-q4-major-linux64.tar.bz2",
"checksum": "SHA-256:96a029e2ae130a1210eaa69e309ea40463028eab18ba19c1086e4c2dafe69a6a",
"size": "99857645"
},
{
"host": "i686-pc-linux-gnu",
"url": "http://downloads.arduino.cc/tools/gcc-arm-none-eabi-7-2018-q2-update-linux32.tar.bz2",
"archiveFileName": "gcc-arm-none-eabi-7-2018-q2-update-linux32.tar.bz2",
"checksum": "SHA-256:090a0bc2b1956bc49392dff924a6c30fa57c88130097b1972204d67a45ce3cf3",
"size": "97427309"
}
]
}
But you can see this starts getting pretty complex. I find the Arduino Boards Manager and boards platform systems to be very interesting, but someone who only wants to get started on their project probably will not. For them, it is probably better to try to troubleshoot the Boards Manager installation problem before resigning themselves to doing a manual installation.