First thing I noticed was the look of the medium.
Loam is a good starting medium.. But it's usually mixed with some sort of other soil to make it full range.. From what I remember, loam can be made up of mostly sand, or silt or clay.. The more silt/clay, the more water it holds. Which looks like what yours is based on the pics. You may need to add some sort of drainage to it, like perlite or something along those lines. Bad drainage leads to stunted growth, compaction and other things.. Ooorrrr, it could more sand based and drain too quickly.. In that case, you'd want to add a compost or some organic matter to help with absorption.
Then there's the ph.. The ph of loam depends on the texture of it. Sandy loam is often acidic with a low ph.. You need to know the ph of that loam..
You also need to know the ph of the tap water that you're using. Bad water ph, plus bad soil ph equals a bad plant.
Next.. there's the feeding. Typical loam has enough nutes for a few weeks, but then needs to be fed. Im not familiar with a water only loam cannabis grow. You will need some sort of fert at some point.
I know it's a lot.. But these are all bare minimum, basic type of things that you will need to think about, or look into.
For a seedling to be 6 weeks old and have only one set of leaves, tells me that there are several issues at play. And they each need to be considered as part of the overall problem.
The lighting might be an issue as well.. I don't like talking bad about any light source, so I won't.. But I will say that the description on amazon says it's 24w total.. It has two fixtures so I'm guessing that the 24w is split between the two fixtures. 24w on the box may not be 24w at the wall. Then there's the efficiency of the light.. I'm assuming it's low efficiency. That could mean that you are getting less than 24 actual watts. Probably closer to 12-15w if it's low efficiency.