Unfortunately, when your image is too low in resolution, we cannot print it as a poster.
This often happens if you try to use an image taken from the web. Web images are very poor quality, what looks good on a screen will not look good printed out, even small, let alone as a large poster. This is because screen resolutions are much smaller than print resolutions. A full screen high resolution photo on your monitor barely has enough resolution (or source pixels) to be printed at regular notebook size.
An image needs to have at least 75 dpi (dots per inch) in resolution at the desired print size. We have a great guide to resolution and file preparation here.
Try finding a higher resolution photo or image to start with. Once you have a high enough resolution image, the order page will allow you to continue and even tell you the largest recommended size for the image.
It's a common next quesiton for someone to say "my image is too low quality, isn't there something you can do?" If there was a way to magically enhance your image, we would sure do it. But unfortunately you cannot "create" pixels that don't exist. That is what causes blurriness. A tiny image can't be enlarged to giant proportions if it doesn't have enough pixels to begin with.