How do you access Mission 46?

According to this link, you are needing to have:

  • Complete all of the main story missions (excluding the repeat missions) before and after this mission.

  • You must also complete all of the important Side Ops indicated in yellow.

  • Listen to all of the yellow cassette tapes

  • You will need a fully upgraded Mother Base and all of its platforms.

If I remember correctly, there are about 49 total missions but Mission 46 marks the end of the story, Truth.

Due to comments, I will probably say a nearly fully upgraded Mother Base instead of a fully upgraded one. Maybe this link could help you unlock mission 46: http://www.gamefaqs.com/boards/718564-metal-gear-solid-v-the-phantom-pain/72466817

The text in it:

To unlock Mission 46 "Truth: The Man Who Sold the World ", Phantom Pain's true final mission you need to meet certain requirements.

- Complete missions 1-31

- Complete postgame Story Missions

    Mission 32 To Know Too Much

    Mission 38 Extraordinary

    Mission 41 Proxy War Without End

    Mission 43 Shining Lights, Even In Death

- Complete postgame Side Ops Story Missions

    Side Op - Extract the AI Pod

    Side Op - Secure the Remains of the Man on Fire

    Side Op - Search for the Escaped Children 1

    Side Op - Search for the Escaped Children 2

    Side Op - Search for the Escaped Children 3

    Side Op - Search for the Escaped Children 4

    Side Op - Search for the Escaped Children 5

- Listen to all story audio/cassette recordings (they are yellow in colour)

- Upgrade entire base, and platform to level 3 or higher.