I have every requirement for the base and I’m level 5 but it won’t let me upgrade the base to level..

I am not sure but you should reach base 6 when you complete all the tutorial stages and prompts. Are there any outstanding activities that the game is asking you to do?

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I had the same problem with the base not upgrading to level 6 what i did was i upgraded the ration truck and when that was done i clicked on the base and tried again and it wasn't greyed out anymore so i got to level the base to 6