Insert ACF Field Variable into Transcoder Shortcode

I’m working on a WordPress site that uses this plugin to convert video files to MP4. The site allows users to post content via an ACF form.

The issue I’m having is I need to pass the ID of the media file into the shortcode. It works fine when I manually enter the ID e.g:

echo do_shortcode('[rt_media attachment_id= 970]');

However when I add a variable to the shortcode nothing is outputted:

$videoid = the_field('select_video_file');
echo do_shortcode('[rt_media attachment_id= '. $videoid .']');

I’m sure it’s got to be something really simple but I can’t workout where I’m going wrong!

Any help/advice would be greatly appreciated.

Thanks in advance,

James