If you are seeing errors from your debugger such as
strace: (PTRACE_ATTACH): operation not permitted
ptrace: operation not permitted
ptrace_attach: operation not permitted
etc.

This is likely because of a linux kernel hardening setting.
/proc/sys/kernel/yama/ptrace_scope
This setting prevents a process from running ptrace on a non-child process.
Even with this on, a can still ptrace another process if it is a child.
Debuggers like gdb and radare2 use ptrace when you attach via PID.

You can turn this off
$ sudo su
$ echo 0 > /proc/sys/kernel/yama/ptrace_scope

Turning this off is global, though.
Instead, set the capabilities of just your debugger to override this setting.

$ sudo setcap CAP_SYS_PTRACE=+eip /usr/bin/gdb
$ sudo setcap CAP_SYS_PTRACE=+eip /usr/bin/radare2