It seems that the software thinks that some of the prices shown to respondents were negative. When it tries to take the natural log of a negative value, it cannot evaluate. Are you expecting that some of the prices you showed respondents via a Summed Pricing attribute were negative?
In order for Log-Linear price specification to work and create a non-linear "bend" to the curve, you need to have specified 3 or more price values in the "Price Values" grid that is shown when you are in the Settings (gear icon) for HB estimation in Lighthouse Studio ACBC HB estimation. From the ACBC/HB Settings dialog within Lighthouse Studio, you click Attribute Coding, then on the Price Attribute, you change the Coding drop-down control to say Log-Linear. When you do that, a dialog appears to the right where you specify which price levels you want to estimate utilities for and to move into the market simulator. By default, only two price levels are shown (the minimum and maximum price levels the software thinks you could have shown respondents). But, you must expand this to include more price points if you want any log-linear term effect (a bend in the curve).
For example, when you open the Price Values dialog, by default it might have two rows:
But, if you want to have a proper log-linear curve estimated and moved into the market simulator, you should expand the number of values to be something like:
With this in place, discrete utilities associated with the log-linear fit to these six price points along the price function will be written out to the market simulator for your use there.