Lighting in a map is not building

Hi!, I have been having this issue for past few days now. I have a medium size level set up and I’m trying to build lighting for it. The light used to build correctly and without any problems. But from the past few days the lighting is not building. It goes to building lighting 0% and then vanishes. The error log says sky sphere has no simple or complex collision. This problem is particular to this level and on my computer only. We have two computers synced with perforce source control. The lighting builds fine on the second computer. How do I fix this?

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