You may see this error when implementing the MAX® 10 Soft TX LVDS IP in External PLL mode, using Quartus® II software version 14.1 and later. The tx_inclock is missing from the RTL source file.
To work around this problem, change the Altera Soft LVDS TX IP to internal PLL mode or enable the "Register \'tx_in\' input port" option on the Transmitter Settings tab of the MAX® 10 Soft TX LVDS MegaWizard™ Plug-In Manager.
This problem is scheduled to be fixed in a future version of the Quartus® II software.