Can't connect tanks

@AndrewGarrison I realised the top part workaround and even a way on using interstages and fuel adapters but forgot to delete the post, sorry

This is due to how attach points work. Fuel tanks have two special attach points. One in the middle that can rotate to any angle and another attach point at the top that can rotate up to around 90 degrees to make a connection. Fuel Tank 18 is oriented so that you are trying to connect its bottom attach point to Fuel Tank 15. The bottom attach point does not allow any rotation when connecting. The must be perfectly lined up to connect with this attach point, or you could flip it around and easily attach the fuel tank using its top attach point instead.