Welcome to the ROOT Forum!
Did you try to search the forum for the same kind of problem? See for example:
https://root-forum.cern.ch/search?q=Error%20in%20%3CTBufferFile%3A%3AWriteByteCount%3E%3A%20bytecount%20too%20large%20order%3Alatest
And in particular: Writing large trees into files
If it doesn’t help, feel free to ask for more help