The goal of this clinical trial is to learn if bitopertin works and is safe to treat EPP or XLP in participants 12 years or older. The main questions it aims to answer are: * Whether bitopertin increases pain-free sunlight exposure after 6 months of treatment in participants with EPP or XLP. * How PPIX concentration levels change from before bitopertin treatment to after 6 months of treatment. Researchers will compare bitopertin to a placebo look-alike substance that contains no drug. Participants will complete daily questionnaires and attend study visits for assessments.