Pins are missing on PCB Altium
My guess is on your schematic, the diode has it's pin designators labelled "a" and "c" for annode and cathode, but the PCB footprint has the designators as something else (maybe "1" and "2", or "A" and "K", etc.). This happens all the time with diodes especially within my company, I can tell you right now.

When the ECO tries to link the two, it looks on the PCB footprint for a designator "a" and "c" and can't find them. Simple as that.

Look at what your PCB footprint for the diode has the designators for the pads labelled as, and correct either the footprint or the schematic.