Why is autoref displaying a blank space in place of the "figure" text when referencing

Whatever else you do, you must delete stepcounter{fig:figmafigure} since this instruction throws an error message.

It would appear that you're using the subfig package. If this assumption is correct, you need to provide the instruction

providecommand{subfigureautorefname}{Figure}

in order to tell autoref which prefix is should use when processing autoref{fig:figmaevolution}. And, do place the subfigures' label directives inside the [...] parts.

That said, you may want to employ the cleveref package and its user-level macro cref instead of autoref. For cref can do just about anything that autoref does, just better. For instance, cref can take multiple arguments, whereas autoref can not; see the example code below for an application.

Incidentally, I can see no valid reason for using doubled sets of curly braces to enclose the includegraphics statements; single curly braces will do just fine. Last but not least, you must place label{fig:figmafigure} after caption{} if you intend the cross-reference the entire figure somewhere in the document.

documentclass{book} 
usepackage{subfig}
usepackage[demo]{graphicx} % remove 'demo' option in real document
usepackage[justification=centering]{caption} % optional

usepackage{hyperref}
providecommand{subfigureautorefname}{Figure} % <-- new
hypersetup{colorlinks,allcolors=blue} % optional

usepackage[nameinlink,noabbrev,capitalize]{cleveref} % optional
begin{document}

setcounter{chapter}{4} % just for this example
setcounter{section}{1} % just for this example
begin{figure}[ht!]     % '[ht!]' is safer than 'H'
centering

subfloat[Evolution of Figma's market share in comparison to its competitors', following its release.%
   label{fig:figmaevolution}]%
   {includegraphics[width=0.475textwidth]{figures/prototype/tools/PrototypingNEW}}% % no need to specify '.png' extension
hfill  % "hfill" is better (and more robust!) than "qquad"
subfloat[Figma's market share for UI Prototyping, in comparison to the market in which it competes.%
   label{fig:figmafourtimes}]%
   {includegraphics[width=0.475textwidth]{figures/prototype/tools/Prototyping2NEW}}

subfloat[Results from each individual surveyed category, showing Figma as clear collective victor.%
   label{fig:figmadominance}]%
   {includegraphics[height=0.55textwidth]{figures/prototype/tools/FigmaDominance}}

caption{} label{fig:figmafigure}
%% stepcounter{fig:figmafigure}  % this instruction throws an error 
end{figure}

A cross-reference to autoref{fig:figmaevolution}.

Cross-references to cref{fig:figmaevolution,fig:figmadominance}.
end{document}