  We consider the most energetically favorable symmetry-allowed spin-singlet
and spin-triplet superconducting pairing symmetries in twisted bilayer graphene
at the magic angle, whose normal state physics is described by a six-band
effective tight-binding model. We compute the Chern number as a function of the
superconducting order parameter strength and the chemical potential and we find
a topological phase transition only for the chiral $p+ip'$ superconducting
state. Different from the regular graphene systems for which this happens at
the van Hove singularity, for TBG the topological phase transition arises at
the point where the Fermi surface becomes tangent to the boundary of the first
Brillouin zone. For each pairing symmetry we study the formation of subgap
impurity states for both scalar and magnetic impurities. We analyze the number
of subgap states as well as their spin polarized density of states that we find
to exhibit peculiar properties that allows one to distinguish between
spin-singlet and triplet pairing. Thus only triplet-paired states may exhibit
opposite-energy impurity states with the same spin, same as for regular
graphene systems\, moreover we find that this spin may flip at the
twist-induced van Hove singularity.
