Why can't this LM324 op amp reproduce a signal above a certain frequency?
It seems like you are running into Slew Rate Limitations, and your output is presenting what is called 'Slew-Induced Distortion' - the Op-Amp's Output Swing is limited by the Slew Rate, so as frequency increases the limit for maximum output swing without 'Slew-Induced Distortion' decreases - typically Op Amps Datasheet have an 'Output Swing vs Frequency' plot.

Have a take a look at Figure 6 of the LM324 Datasheet, and where your signal is in the plot according to the scope captures you shared (see below). Ideally you'd want to stay "under the curve".

If you want to learn more about Slew Rate take a look at the 'Slew Rate' series in the Precision Labs for Op Amps training.