Size of Output vector from AvgW2V Vectorizer is less than Size of Input data
I think the issue can be one of the two :

A . You have missing value in x_train

B . One of the values in x_train has no word that is there in modelwords.

In both the cases ,

if everyword in modelwords:
     w2v = training_model.wv[everyword]
     count += 1
     sentence += w2v

condition doesn't get satisfied and you end up not addding any new value to sentence