You should NOT need a decryption key to download from MEGA. If MEGA wants you to enter a decryption key, do the steps below to fix.
⦁ Choose the "Skip this message next time", click Continue. Close the new MEGA tab if it requires a decryption key.
⦁ On justpaste.it site, click the MEGA link again. The problem should be gone now and you're ready to download.