Why does this come up?

Possibly because your GPU is already discontinued. You can try going to Ae's preferences in the Display tab and see if you can turn it on. We'll have a better overview when you give us more technical information about your system.

Aunque no nos das ningun detalle de tu proyecto o de tu sitema, lo primero que deberias hacer es tratar de actualizar  la tarjeta grafica, y si ello no soluciona el problema, deberias darnos mas detalles sobre tu pc o mas a fin de que podamos ayudarte.
saludos

Without any info about your system, your settings, your project contents and so on we can't tell you why you hardware acceleration may not be working. You have to be much more specific. Start with the usual standard troubleshooting: Update your system's graphics driver, check the relevant settings in AE.

Mylenium