acf function shows error while fiiting time series
Logarithm cannot handle numbers less than and equal to zero. You can exclude them from your time-series and then add them to the graph as points with different colors (if they are not many that would make the graph ambiguous and interpretation of it hard) or just simply exclude them but have the original graph side by side as well.

EDIT:

As proposed in the comments this would be a better option to avoid zeros:

log⁡(x+1)

or for negative numbers as well:

log⁡(|x|+1)