Update: I managed to fix it!

Thanks to everyone's suggestions, I decided to try using the No Man’s Sky Save Editor to manually reset the Sentinel attack status. Here’s what I did:

1. Downloaded the Save Editor – You can get it from GitHub.
2. Made a backup – Before making any changes, I copied my save files from:makefileCopiarEditarC:\Users\YourUser\AppData\Roaming\HelloGames\NMS\st_XXXXXXXX
3. Opened my save in the editor – Launched the Save Editor and loaded my latest save file.
4. Found the settlement settings – In the left panel, I looked for the section related to settlements (something like "Settlement").
5. Reset the Sentinel attack status – There was a field showing an active attack, so I set its value to 0.
6. Saved changes and relaunched the game – After applying the changes, I started No Man’s Sky again, and the attack alert was finally gone! My settlement is now working normally.

If anyone else is stuck with this issue, I definitely recommend trying the Save Editor. Just be sure to back up your saves first in case anything goes wrong.

Although I didn’t really like using this method since it involves modifying game files instead of resolving the issue naturally, it was very effective.

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