Hi Carlos,
The hard coded grouping in the XOG request doesn't match the group of the existing plan. If you're willing to share this process we can probably suggest a fix, other wise please contact me, your technical adviser or a partner.
The gel process needs to read the grouping of the plan then write that back in.
Here is an example of the bare minimum to update a custom cost plan attribute
<NikuDataBus xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="../xsd/nikuxog_costPlan.xsd">
<Header action="write" externalSource="NIKU" objectType="costPlan" version="13.3.0.286"/>
<CostPlans>
<CostPlan code="COST_PLAN_ID" investmentCode="PROJECT_CODE" startPeriod="start_period" finishPeriod="finish_period" name="project_name" periodType="MONTHLY">
<Description/>
<!--this is the part you need to get right as per current plan-->
<GroupingAttributes>
<GroupingAttribute>transaction_class_id</GroupingAttribute>
</GroupingAttributes>
<CustomInformation>
<ColumnValue name="my_custom_field">what_is_going_to_update</ColumnValue>
</CustomInformation>
</CostPlan>
</CostPlans>
</NikuDataBus>