How to find the PPSSPP saves in Ubuntu

Since the only way to run Terraria on Ubuntu is through WINE as Ben Brocka and lunboks have suggested, I will answer your actual question, assuming that you are using WINE and if you are not, I would suggest that you revise your question to provide more information about your setup.

While I'm sure you knew this if you backed up your files, but to provide background for the answer, as per this question, the default behaviour is for saves to be in My Documents\My Games\Terraria\Players and worlds to be in My Documents\My Games\Terraria\Worlds as stated here.

You will want to put your character and world files in the paths mapped for My Documents\My Games\Terraria\Players and My Documents\My Games\Terraria\Worlds respectively. The default WINE behaviour if I read correctly is for My Documents to map to your home directory so it would be home/(user)/My Games/Terraria/Players and home/(user)/My Games/Terraria/Worlds for your respective username I believe. If you want and this is up to you, you can remap where My Documents maps to in wineconfig, but you will have to adjust where you put the save files accordingly.

I made some tests and Bryan C. comment's url confirmed my theory, according to this:

A virtual memory card is created automatically when you start PlayStation® format software and save your game. For each application, two virtual memory cards (for MEMORY CARD slots 1 and 2) are created with the same name as the title of the game.

So my theory was that when you play a game on PSP, it looks like it just creates the memory card which you saved on it, while on PSVita it expects to have both created on start.

So what did I do? Went back on PSP, loaded my save (Slot1) and then saved once on Slot 2. Then tried copying from PSP to PSVita and voilá, it ran without errors.

TLDR; To fix 'Cannot access the memory card' error when copying a PSOne Classics savegame from PSP to Vita, simply open the save on PSP/PS3 and save on both slots 1 and 2 and then try copy again to Vita.

Of course if you don't have a PSP/PS3 anymore, I still don't know what you should do