Hi,
I'm using version 1.01 1402 dev. 17-may-21.
Went into figure mode, deleted t.rex object. (model), added t.rex again, for a fresh start on skinning.
The left leg is now working fine, using influence volumes and also with weight painting.
Try removing t.rex model from figure mode and adding again. (that does mean skinning again as well)
If that doesn't make a difference, try a different build of anim8or. It defineately works ok with the build i'm using.
Open new version of anim8or. Import figure from your current anim8or file. Delete the t.rex model in figure mode, (keep rig), then add t.rex object again. (figure mode -> build -> add object )
Whether you choose to skin with influence volumes or painted weights is your choice.
Influence volumes can be easier with gr8 results, painted weights allows attaching the model to the rig with more pinpoint precision. (literally, can attach one point/vertex at a time. Can be really handy)
Good luck with your video plan. Sounds good.

Edit: make sure that you add t.rex object to the middle bone, protruding from the root.
When i opened your project, it had the fault you've shown. I fixed it Using the same version i opened it with originally, so rather than being an anim8or fault, perhaps you 'accidentally did something not quite right'.
It happens.