So I was making a huge reactor in a creative world and when I tried to complete it it said a block was not a part of the multiblock, and when i checked, it was fuel cell, I deleted it and it moved on to another fuel cell. I don't know what my problem is.

How big is that reactor? With default configs, you can only build reactors up to the size of 24x24x24. If your reactor is bigger than that, either you need to change your reactor or change the configs for NuclearCraft Overhauled.

It is within size parameters, I already had that problem and solved it. While i was building it initially on the inside i was getting lag spikes, which meant the multiblock was updating. At some point it stopped lagging, which meant it wasn't updating anymore, I just don't know when because I was not paying attention.

@Elibikerdude Is that cell connected to any other heat-conducting multiblock components? If not, it is not considered part of the multiblock, and even if it was, it would melt as it would not be able to have its heat extracted ;)