import

To Develop a selected image in Lr-Classic I use the keyboard [D] key, or click on the top Menu 'Develop'.

A [Double-Click] only puts me in Loupe View.

"File Cannot Be Found"  commonly means the photo is shown as 'Missing' with an exclamation mark on the grid view preview border, or a question mark on the containing folder. Lr-Classic has lost the path to the original image file. Several reasons can make photos go 'Missing'.

MISSING FILES AND FOLDERS-(Computer Darkroom)

After-thought:  Your post header says "import".! You cannot go directly from an Import dialog (showing photos to Import) to the Develop module. You need to complete an Import process for the photos to be added to the library, then you can Develop.