Easy Way: Update your nvim version.

Hard Way: Replace all the bufnr with buffer

Explanation: Neovim latest uses buffer instead of bufnr. Your custom config (plugins or any file) which are updated are looking for buffer but found bufnr. So considers bufnr as function.

What I did in Linux to not mess with my custom config? I installed the latest nvim and set the path in my ~/.zshrc or ~/.bashrc file.