When you have missing observations on the RANDOM _RESIDUAL_ statement in GLIMMIX then the procedure generates the warning message. Considering change the RANDOM _RESIDUAL_ to -
Proc glimmix data = JessicaPCR;
Class ID TRT BLOCK DAY;
Model AMPK = TRT|DAY/ddfm = KR;
random intercept / subject=BLOCK;
random intercept / subject=ID*TRT;
random day /subject = ID*TRT type=arh(1) residual;
LSMEANS TRT|DAY/ pdiff lines;
nloptions tech=nrridg;
Run;
The above RANDOM statement should make note go away.
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