LTSpice Error: Timestep too small error
Like mentioned in the comments, your source is too ideal and creates a very extreme and impractical \$\frac{dV}{dt}\$.  That ideal triangle shape you're simulating will also never show up in real life.  I've done DO-160 Lighting Tests, and their waveform specifications always have some kind of RC smoothing on them to be more realistic.
I was able to get your simulation to simulate using a slightly less intense waveform via an RC and also forcing the maximum time step size to 1p (see the .tran statement).  But keep in mind this takes a long time to simulate.



Since you didn't explain what you're trying to measure, I have to assume the following might be important too.  If you need to avoid current limiting via loading from that series resistor, then you can pass the smoothed waveform through a Gain=1 voltage buffer like so: