cannot find files to convert

i just updated to itunes 12.7 and after i import mp3 files successfully and they play fine but when i got to convert them to AAC which is highlighted it says files have not been converted because they cannot be found

Rather than import the MP3 then converting, do it in 1 step.

Hold Option then menu File > Convert > Convert to AAC and select the files or the folder you want to add and convert.