octomap_server

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Hi, i am very new to octomap, so my question is probably very basic, but i couldnt find a solution by myself. I am not using any laserscan yet, only an already existing .bt file i want to load into RVIZ. From what i have seen so far, i need to run "rosrun octomap_server octomap_server_node <file.bt>", but this doesnt work somehow. I am constantly receiving the same error

[ WARN] [1670241286.185374293]: Nothing to publish, octree is empty
ERROR: Filestream to car.binvox.bt not open, nothing read.
[ERROR] [1670241286.196390994]: Could not open file car.binvox.bt

Do i need to include some code somewhere to open the .bt file, if yes where should i include it and how? Or do i need to edit the octomap_mapping.launch file? I tried this too, but there are too many scanner related parameters i didnt know what to do with them.

Thank you all for your help!