Your syntax {t, 0, 12} is typical of NDSolve, not of DSolve. Unfortunately, NDSolve does not support CaputoD at the moment, apparently. DSolve does support it, but in a very limited way: try these two independent equations, that DSolve can solve separately but not together:
eq1 = {CaputoD[S[t], {t, 90/100}] == S[t],
S[0] == 1};
eq2 = {CaputoD[iI[t], {t, 90/100}] == 3,
iI[0] == 1};
DSolve[eq1, S[t], t]
DSolve[eq2, iI[t], t]
DSolve[Join[eq1, eq2], {S[t], iI[t]}, t]