You're using a non-English language version of Morrowind. Morrowind's cell names are their IDs unlike later games, where their IDs are just some hexadecimal. This means the script is looking for "Solstheim, Mortrag Glacier" when there's only "Solstheim, Glaciar de Mortag". Sorry, but Morrowind just hates other languages. I'd reccomend making a separate English install and make a new character for that so that you can play TR.

If you want to get dicey and fix this script here's how:
1. Install Wrye Mash, Polemos Fork and use it to copy TR_Mainland.esm to an esp (once installed right click it and the option appears)
2. Install the Construction Set if you don't have it in your Morrowind folder already
3. Open your Morrowind.ini, find the [General] section (usually on the very top), and at the bottom of it paste in the line "allowyestoall=1"
4. Open the Construction Set, File->Data Files->Double click TR_Mainland.esp->Set as Active File->OK
5. While loading it will throw up a bunch of meaningless messages about dialogue being edited, click "Cancel", if it makes more errors like what you have take note of them and repeat steps 6 through 10 but with the new cell names and script names.
6. Once it loads, confirm what the cell name like "Solstheim, Glaciar de Mortag" is really called by clicking in the left part of the bottom right window and typing in "so" and then scrolling until you find it.
7. Click the pencil in the row under the one with File and such, Script->Open->tr_m3_OE_MGGemReward_sc
8. Copy/paste the whole script into a text editor (say Notepad or Notepad++, probably not Word)
9. Use that text editor's replace function to replace "Solstheim, Mortrag Glacier" with "Solstheim, Glaciar de Mortag" (or whatever it is named)
10. Replace the entire script with your edited version, click the Save button under the word Script, and then the Recompile All button next the the red X in the same row as the Save button.
11. Close the Script editor, save (Ctrl+S) and close the Construction Set
12. Reopen Wrye Mash, right click TR_Mainland.esp, and click "Copy to esm"
13. Open Morrowind and see if you get any more of these kinds of errors, if so repeat steps 4 through 12 but with the new cell names and script names.

Hopefully if you want to edit the esm itself, my instructions work, but again, I am sorry for how Morrowind treats other languages.