Try this.
It seems you forgоt declare the p.
syms z p r=2.5e-6/2; alpha= acosh(z/r); A= (2*sinh(((2*p)+1)*alpha)+(((2*p)+1)*sinh(2*alpha))); B=power(sinh((p+0.5)*alpha),2); C= power(sinh(alpha),2); D= power(((2*p)+1),2); E= (4*B)-(C*D); F= (p*(p+1))/(((2*p)-1)*((2*p)+3)); G(p,z)= F*(A/E); s = symsum(G(p,z), p, [0 inf]); pretty(s);