[ASK] Could not calculate the upgrade di 12.04

[Edogawa]

Maaf mimin mau tanya,
ini saya mau upgrade dari 12.04 LTS ke 14.04 LTS.

namun ketika proses calculate muncul pesan error seperti ini :
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Could not calculate the upgrade

An unresolvable problem occurred while calculating the upgrade.

This can be caused by:
* Upgrading to a pre-release version of Ubuntu
* Running the current pre-release version of Ubuntu
* Unofficial software packages not provided by Ubuntu

If none of this applies, then please report this bug using the command 'ubuntu-bug ubuntu-release-upgrader-core' in a terminal.
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Ini maksudnya apa ya? Pengguna baru Ubuntu jadi kurang ngeh. Mohon informasi dan solusinya.

Sebelumnya terima kasih.

[thrvers]

'
perintah upgrade precise ke trusty-nya seperti apa? kok keluar pesan tsb