There is a reaction force even though there is no boundary condition at the node.
You need to check if there is enforced displacement or settlement applied to the node.
Sometimes, even if there is no boundary condition at a node, reaction forces can be generated when checking the reaction forces after analysis. The reason for this is that enforced displacement or settlement was applied to the node.
In midas Civil, enforced displacement is considered as a load condition. When an enforced displacement is applied to a node, the degree of freedom corresponding to that displacement is automatically constrained, and the node behaves like a support for other loading conditions that do not consider the enforced displacement.
Defining 'Settlement Group' for a node to have settlement in the DZ direction, which does not have any DZ constraint, will automatically constrain the node in the DZ direction during analysis. This constraint condition affects not only the Settlement Load Case but also all defined Load Cases / Load Combinations.
For example, if a simple beam with roller and hinge boundary conditions is modeled at the start and end points, but when checking the reaction forces after analysis, it is found that there are reaction forces at points other than the start and end points (nodes with enforced displacement and settlements).