I just had a chance to check out the .blend file!
The older Elbeem simulator, Mantaflow in 2.82+, and the FLIP Fluids addon are grid based simulators so they have some things in common for how the domain works. The domain is split up into a grid of voxels where the Resolution value controls how many divisions are on the longest side of the domain.
The size of a voxel can be thought of as the minimum amount of physics detail that can be resolved in the simulation, similar to how a 2d pixel is the minimum amount of detail in an image. If objects such as inflows are smaller/thinner than the size of a voxel, it may not be resolved effectively in the simulation and may not emit fluid. Here is a visualization of the grid (top view and side view) using the FLIP Fluids addon:
It looks like the inflow is larger than a single voxel, so it should be able to emit fluid.
There is another problem that is causing fluid not to emit. I am not sure if this is true for Mantaflow, but the FLIP Fluids addon and Elbeem simulators have a thin layer of solid voxels at the boundary of the domain. These solid voxels are what keeps the fluid inside of the domain. In the above image, the yellow boundary visualizes the solid layer. Fluid is only able to exist inside the empty space of the yellow boundary.
It looks like the problem is that the domain is too thin on the Z axis and this is causing there to be no empty space inside of the domain. This can be fixed by either increasing the Z scale of the domain, or by increasing resolution which will make smaller voxels and a thinner solid layer. You will likely want to increase domain resolution to add more detail to the fluid simulation.
If you start with a small domain size and then later want to resize the domain to add more area for the fluid, it is a good idea to also increase the resolution to add more voxels. If the resolution is not increased when making the domain bigger, the voxels will end up stretching larger and the simulation will lose detail compared to the smaller volume.
If using the FLIP Fluids addon, there is a ‘Lock Cell Size’ option which can be used to automatically increase resolution proportionally when resizing the domain so that voxels remain a consistent size. World scale (physical size of the domain) is also important to adjust when resizing the domain. The FLIP Fluids addon has a ‘Relative’ world scaling mode that automatically increases the scale proportionally.
This may be caused by a bug in the Mantaflow simulator. I have seen comments on a common issue where Mantaflow can stop responding to changes in the simulation setup or not updating the cache, but I am not sure of a fix for this.
Hope this info helps!