The problem is when P1 choose $A$, they play a game as described in normal form, and when P1 choose $B$, then P2 can choose $a$ or $b$.
I think the Nash equilibria of the Game are $\{U,L\}$ and $\{M,C\}$.
Looking at the subgame that P1 chose $A$, both equilibria gives the payoff to P1 less than the case when P1 chose $B$.
Also, looking at the subgame that P1 chose $B$, P2 will choose $a$.
Thus, I think $\{(A,U), L\}$, $\{(A,M), C\}$, and $\{B, a\}$ are Nash equilibria, and only the $\{B,a\}$ is the subgame perfect equilibirum. However, the problem says "find two subgame perfect equilibria." Are there any wrong things?
This game has two (pure-strategy) sub-game perfect equilibria that induce the same equilibrium outcome: $\{(B,U),(a,L) \}$ and $\{(B,M),(a,C) \}$. The first pair in each equilibrium specifies player $1$'s strategy while the second pair specifies player $2$'s strategy (in hopefully the obvious way).
Recall that an equilibrium is always a profile of strategies, and the strategies are fully-contingent plans that determine actions even off the equilibrium path.
In particular, as I suggest in my comment above, $\{B,a\}$ is an equilibrium outcome but not an equilibrium. It does not specify what actions players $1$ and $2$ take in the event that player $1$ initially chose the action $A$ (even if we know this will never happen in this equilibrium).