I need some more information in order to recreate the issue. Have a look at what prerequisites plutonium synthesis has. Create a new game and have another look at it's prerequisites. If the prerequisites are exactly the same, please save the new game then upload it for me? I'll then be able to sync my mods and settings to that save game file.
If you need somewhere to upload the new save game, create an issue on the GitHub: https://github.com/modded-factorio/bobsmods/issues/
A screenshot of the tech tree would also be useful.
