Hello, you can check this help from Cura support: https://support.ultimaker.com/hc/en-us/articles/360014055959-Why-am-I-getting-Your-model-has-missing-or-extraneous-surfaces-popups-in-Ultimaker-Cura-
Try to zoom and rotate around the model to find the nonmanifold part or you can use other software to fix the nonmanifold part automatically if you couldn't find it.