PCIe doubt

Hello,

When I export my "txs" port from PCIe I get the error "txs must be associated with an associated Clock" What should I do to eliminate this.

Why do I get error when I connect the "tx" of one pcie system [the tx conduit is exported] to the "rx" of another pcie system [the rx conduit of this system is exported, and I am connecting the exported ports together in GUI].

Thanks