In Jain philosophy, the main mistake of the Ajivika doctrine is that it does not accept the full Jain understanding of soul, karma, and liberation.

From the Jain view, Ajivikas were mistaken especially because they believed in strict fatalism—that everything happens only by destiny and that personal effort, vows, right conduct, and spiritual discipline have little or no real role. Jainism teaches the opposite:

• Jiva (soul) exists
• Karma binds the soul
• Right effort, austerity, discipline, and conduct are necessary for liberation
• Moksha is attained through purification of the soul

So, from Jain Dharma, the error of Ajivika belief is mainly denying the importance of purushartha — conscious right effort — and not fully accepting the Jain path of Ratnatraya: Samyak Darshan, Samyak Jnan, Samyak Charitra.

If you want, I can also explain the difference between Jainism and Ajivika in simple points.