I just created the following output with QNEAT3 Iso-Area as polygons (from Point) - as you can see, in my case it works. So it must be a problem with the settings or the input: What fieldtype is your speed field? I don't know if the algorithm accepts real (decimal) numbers, I used integer. Does every segment of your network layer have a speed value? Maybe some of the values are 0 or NULL or otherwise invalid/empty?

In the screenshot you can see that I deleted the default speed value, not sure if that affects the algorithm. 

Maybe also an issue with the projection? I used a projected CRS.

Try if any of these hints solves the problem and come back to report if not. Without more details it's difficult to say what causes the problem. Maybe a screenshot, input-parameters or a sample of the data you use could help to further investigate the problem if the suggestions here do not help.

See my settings and output (viridis color ramp: Output interpolation raster; white lines: Output Polygon isochrones; black lines: input network with speed attribute).

[![enter image description here][1]][1]


  [1]: https://i.sstatic.net/wJVqC.png