You may see this error message when compiling a project which instantiates the Intel® eSRAM IP followed by information messages similar to these refering to the input ref clock :
Error(175001): The Fitter cannot place 1 pin, which is within <esram_name>
Info(14596): Information about the failing component(s):
Info(175028): The pin name(s): <esram_refclk_name>
The reason this fitter error occurs is because the eSRAM input PLL reference clock does not have an LVDS I/O standard assignment in the project.