Columns cannot be merged if they belong to parent/child groups. If column groups are adjacent and static, then you can merge them through the options in the cells' Context Menu.
If you need further help, please paste the code for generating the report and include details which columns you need to merge. Note that code files cannot be attached to forum posts and if you want to share the project, we recommend the usage of the support ticketing system.
Regards,
Stef
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