Vivado automatically derives clocks on the output of a CMB based on the CMB settings and the characteristics of the incoming master clock. If you define a primary clock on the output of a CMB, Vivado does not auto-derive a clock on the same output.
This check reports that a primary clock has been created on the output of a CMB. This breaks the relationship with the incoming clock and prevents accurate clock insertion delay computation. Defining a primary clock here is not recommended because it can cause inaccurate timing analysis hardware failure.