New Guys cant print photos

Are you looking to print the image to fill the printed page? If so, you will need to change your print driver settings (from within the LrC print module), not only just to the right paper size, but also, to a Borderless mode - if available. The print driver then communicates to LrC that its hardware printing margins (that are currently taking up space on the page and limiting how much you can enlarge the image cell), are now slimmed down to zero. So that will permit you to enlarge the image cell right up to the paper edges.

Depending on printer, there may be a 'true to scale' or 'don't enlarge' borderless option. But more typically, images printed borderless are enlarged by a few % within the print driver / the printer. This ensures no white sliver edges are left due to paper size inaccuracies or misalignment. But it also means, a part of the picture that is presented very close to the paper edge in LrC, is in practice going to get lost as overspray / bleed allowance. Because LrC does not preview borderless mode image enlargement. And that enlargement always happens with that printing mode. Only without that, is your printing going to be true-to-scale. But it's worth remembering, seldom will the printing paper itself be manufactured exactly to the stated dimensions anyway, nor get loaded in exact alignment.

If OTOH you are expecting to leave a white uprinted border on the page, and don't need 'borderless', then this may be more an issue of how the image "fits" inside its containing cell. You have the option to make the photo 'entirely fill', rather than 'fit'. When filling the cell, an on-the-fly, additional crop is imposed dynamically: you don't need to have set the image's main crop / aspect to match this particular printing cell shape.

If you don't like the result of this 'fill crop' compositionally, you can slide the image right within the print module - or else go back to Develop module and update its main crop's aspect ratio.

LrC lets you display dimensions of each print cell so that you can view the aspect ratio inside that, if you did want to match those proportions in your image crop aspect ratio.

Sorry, I'm getting confused by switching between inches and cm's.

Let's drop templates for now.

Here is what I see when I'm trying to print a 4x6. I've selected the paper and set the paper to be 4x6. I have not selected any template and Single Image. The margins are what LR allows as a minimum. The Cell size is the maximum. I had white space because the image is a custom crop and not in 4x6 ratio.

Does this help?

Here is my advice-

Do NOT select any of the default Lightroom templates!!! They are ALL for  'Letter' size paper (8.5x11inches).

eg. the (1) 4x6 template is for a 4x6" CELL on paper that is 8.5x11"

eg. The (1) 5x7 template is for a 5x7" CELL on paper that is 8.5x11"

Take the advice of  @DS256  and do your own paper size setup in the printer dialog [Page Setup]. And do not click on any default template again!

When you have your print & paper correct- create your own Print Template saved in 'User Templates' .

Collapse the [Lightroom Templates] group and don't look there again 🙂

Are you setting the correct paper size in 'Page Setup...' (bottom right in Print module). It sounds like you have a large paper than 4x6 selected.