Q.Butternut squash
Plant raised and kept indoors are starting to turn yellow and die, why?

Your seedlings may not have had enough sunlight.
Another cause of yellowing leaves is watering issues; either too much or too little.
Here are some links with more information.
https://www.gardeningknowhow.com/garden-how-to/seeds/seedlings-turning-yellow.htm
https://www.gardeningknowhow.com/garden-how-to/seeds/seedlings-after-germination.htm
https://www.gardeningknowhow.com/edible/vegetables/squash/growing-butternut-squash.htm
https://www.gardeningknowhow.com/garden-how-to/seeds/how-to-harden-off-your-seedlings.htm