Click on the two inputs of the AND2P gate. When you click on the top input of the AND2P gate, NET: CLK will appear in the lower left corner of the design analyzer window. This is the clock for the system. The problem that you are encountering is that the clock is gated. Unless the clock input of the flip flop is controllable from the primary inputs during testing, the flip flop will be untestable.
Click on the bottom input to the AND2P gate from the design analyzer menu and select VIEW > ZOOM OUT until you can see the whole design. As you can see, the other input comes from the primary input GATE.

Figure 11
To fix the testability problem, we will use the SET_TEST_HOLD command. This command allows us to hold a primary input at a constant value during testing. From the design analyzer menu, select SETUP > COMMAND WINDOW. Type in the command set_test_hold 1 GATE as shown in the figure below and press return. By setting the GATE input to 1 during testing, the CLK signal will pass through the AND2P gates to the clock inputs of the flip flop.

Figure 12
From the Test Synthesis window, run CHECK_DESIGN by selecting the CHECK_DESIGN button. This time you should not get any errors.

Figure 13