Warning message during Clock Tree Synthesis

I am new to EDI system. When I try to do CTS, I get a warning message like this. Can anybody please explain what this warning is?

**WARN: (ENCCK-157): Cell SC42CKBUFBCLXH1 is set as dont_touch in the timing libraries. May have to use 'set_dont_touch [get_lib_cells SC42CKBUFBCLXH1] false' in order to delete the buffers in clock tree.