Differential pair in Altium
Your differential pair nets must be named the EXACT same except for the _N and _P at the end. Yours are different--You have PCM_SPK1P_UG96_P and PCM_SPK1N_UG96_N. The "SPK1P" and "SPK1N" need to be the same (i.e. "SPK1"). 

I suggest naming your nets as follows:

PCM_SPK1_UG96_P

PCM_SPK1_UG96_N