This part need to be bought at the R&D center

[Tornado77]

Hello I started a new hard campaign with no mods except visual mods. I need to bought part i research before I can use them but I have a problem, I tried to buy the FL-T400 tank 4 time but its not working it still telling me I need to buy it, also size 1 fuel tanks still have the old texture, when I place them they have the new texture but when I launch they have the old textures. I'm playing on a fresh install. Can someone help me please ? 

[4x4cheesecake]

That's an uncommon issue which can happen during a game update.
The "old" parts are not moved to the "deprecated" folder so the game loads two version of the same part which share the same name. KSP gets confused by two parts sharing the same name, so every time you buy one of these parts, the game will unlock the new version but it displays the old version instead, which will stay locked/unavailable.

The easiest way to fix this, is a clean reinstall of the latest version of the game (or the version you want to run). Create a backup of you savegame, uninstall the game (evtl you have to remove some remaining game files manually), then reinstall it. After restoring the backup of your savegame, you should be able to unlock these parts

[Tornado77]

I just moved my KSP folder somewhere else, re installed the game and I replaced the squad folder now everything work fine !