If you're seeing a few of those every now and then, nothing to be alarmed about. It can happen when the CPU is stressed and is normal. If you're constantly seeing it then you might want to consider either reducing the CPU load or disabling NOHZ.   For understanding what NOHZ does, I highly recommend this Stack Overflow answer to: "How NOHZ=ON affects do_timer() in Linux kernel?".   If you want to get rid of the message, you can simply add the following to /boot/armbianEnv.txt