Rigid body motion means a body translates or rotates without producing any strain. It can happen to both flexible and rigid body. If you have a rigid body motion in simulation, it means some DOF is not properly constrained. For example, in your bike model, the front fork can rotate in the local z-axis, and if you don't apply remote displacement to control the Rz DOF, it will show you a message "Rigid body motion" or "Small pivoting term in the matrix".
Back to your question, about the warning about "remote boundary conditions may conflict with another", here is an explanation: https://ansyshelp.ansys.com/account/secured?returnurl=/Views/Secured/corp/v201/en/wb_sim/ds_one_or_more_mpc.html?q=remote%20boundary%20conditions%20may%20have%20conflicted%20with%20another. It can mean more than one MPC or remote BC is defined on the same face or body.
If you have trouble viewing the above link, please refer to this post: https://studentcommunity.ansys.com/thread/how-to-access-the-ansys-online-help/
This is off-topic, but for your simulation, you may also use "joint" to simplify the simulation process: https://ansyshelp.ansys.com/account/secured?returnurl=/Views/Secured/corp/v195/wb_sim/ds_Joints_types.html?q=joint%20types
Further questions? Please let me know.