One option is to save the project on the old computer again via
File > Save project as with the following options checked at the bottom of the Save-window:
- Create subdirectory for project
- Copy all media into project directory
This way all media files used in the project are copied into a single project directory which you can then transfer over to the new computer.
In general, you can also check and manage source media paths via
View > Project Media/FX Bay:
https://www.reaper.fm/videos.php#U8_fSWVH4qs
In this case you'll usually have to do a general search for the missing file (for instance "2 Plates 01 Bright Plate M-to-S.wav") on your old computer first. Then copy the audio folder or files to the new computer and relink them **.
** For instance via the Project Bay mentioned above. Or via the missing files window shown at project load:
https://reaperblog.net/2015/02/video...offline-files/