"Insufficient number of observation" for lag test

[wuzhuo02]

I have a monetary demand function model data, but when I estimate my model with all of my variable(in fact, my variable is only 2.) Because I running it with the equation of like Y-Y(-1)=c+a(X-X(-1)
When the lag is four, there was an e-views error stated "insufficient number of observation".
What's worse，now I modeling Y-Y(-3)=c+a(X-X(-3), the error occurs. What should I do?
My obs number is 55.
If I did not make my puzzle clear? or you can not follow me quite well, please tell me, anyway, this is the first time I submit post here.

[EViews Gareth]

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