You are getting that error because you use an older version of bash (3.2.25).

Since Bash4, there's &>>TARGET, which is equivalent to >> TARGET 2>&1.

Source: Appending redirected output

So, you should take in consideration an upgrade. I use bash version 4.2.45 and echo "hello" &>> file1 works like a charm for me.

You are getting that error because you using an older version of bash (3.2.25).

Since Bash4, there's &>>TARGET, which is equivalent to >> TARGET 2>&1.

Source: Appending redirected output

So, you should take in consideration an upgrade. I use bash version 4.2.45 and echo "hello" &>> file1 works like a charm for me.