When trying to use the ocd version of the IO cells, I see this error in CTS:
[INFO CTS-0007] Net "clk_PAD2CORE" found for clock "clk_PAD".
[INFO CTS-0122] Clock net "clk_PAD2CORE" is skipped for CTS because it is not connected to any output instance pin or input block terminal.
[WARNING CTS-0083] No clock nets have been found.
[INFO CTS-0008] TritonCTS found 0 clock nets.
[WARNING CTS-0082] No valid clock nets in the design.
I've confirmed it happens on the default template too. There it doesn't cause the build to fail - presumably the tooling can cope without needing a clock tree for a small number of registers, but it's obviously bad, and in a larger design it does cause failures.