It looks like your soil is too wet, and the seedling is damping off. It’s best if you join the forum, and fill out a support ticket. That way we have all the specifics to help you as best as we can. It sounds, and looks like your environment is under control. How are you germinating the seeds? Directly in those pots? How often and how much are you watering, and with what type of water, at what pH?
To create a grow support ticket we need some basic information to help diagnose the issue.
COPY/PASTE the below list into your forum post.
Answer these simple questions the best you can.
If you do not know, or do not use something; Just say so = NA
Strain; Type, Bag seed, or NA
Soil in pots, Hydroponic, or Coco?
PH of runoff or solution in reservoir?
What is strength of nutrient mix? EC, or TDS
Indoor or Outdoor
Light system, size?
Temps; Day, Night
Humidity; Day, Night
Ventilation system; Yes, No, Size
AC, Humidifier, De-humidifier,
Co2; Yes, No
If you can, upload a picture of your plant. It helps the diagnosis, if needed.
Add anything else you feel would help us give you a most informed answer. Feel free to elaborate, but short, to the point questions and facts will help us help you