Install QNEAT directly from the QGIS Plugin Manager (Plugins > Manage and Install Plugins…, search for “QNEAT”).
As of version 4.0, QNEAT no longer depends on the matplotlib library for contour creation. Instead, QNEAT uses the GDAL library for contouring and euclidean distance interpolation which is shipped with every QGIS installation. Every algorithm, including Iso-Area generation, is available out-of-the-box with no extra setup.
Still running the legacy QNEAT3 plugin? Its Iso-Area as Polygon/Contour algorithms required a manual matplotlib install — see the archived QNEAT3 page for those instructions.
When running a QNEAT algorithm it can be useful to enable the processing window to stay open after the algorithm is finished. With this option enabled, you can inspect the algorithms messages and infos that are logged at runtime in the log window. You may set this option under Settings>Options...>Processing>General>Keep dialog open after running an algorithm (tick the checkbox to enable this option).