I have a Bode plot for a low pass filter simulated in LTSpice

There is no -3db point to calculate cutoff frequency because the curve starts at -8db

What should I do?

 

Remember all dB measurements are relative measurements, thus the -3dB point is not an absolute 3dB, it's 3db below the low frequency response.
So if the low frequency response is -8dB (relative to the input) then the corner (cutoff) frequency is -3db below that or -11dB below the input.
You measure this easily by clicking twice on the graph output to get two cursors.
Move one to the low frequency response and then move the other until the difference is shown in the pop-up window as -3dB.

You can also increase the input source voltage by 8dB (a factor of 2.5119) so the low frequency output will be 0dB and then the cutoff frequency will be -3dB from that.