Add ;/& terminators (and other syntax) on the line with the <<, not here.

Problematic code:

sudo bash -s << "END"
  cmd1
  cmd2
END &

Correct code:

sudo bash -s << "END" &
  cmd1
  cmd2
END

Rationale:

You are using &, ;, &> or similar after a here document. This is not allowed.

This should instead be part of the line that initiated the here document, i.e. the one with the <<.

If it helps, look at << "END" as if it was < file, and make sure the resulting command is valid. This is what the shell does. You can then append here document data after the command.

Exceptions:

None.