\Factorio\mods\Power_Armor_MK3_2.1.03.zip doesn't match the expected Power Armor MK3_2.1.3.zip (casesensitivel)

Whereabouts did you download the mod from? Here, in-game or the Github depo? The only reason you should be getting that error is if the info.json reads "version":"2.1.3" instead of "version":"2.1.03". As far as I can tell all three sources are fine, so I'm not sure where the issue came from.

I downloaded the mod from this page, and both the mod file name and the version number in the JSON file are 2.1.03. So I don't know why the game will pop up an error message of 2.1.3. Maybe the version number doesn't support numbers like 03? The meaningless number 0 before 3 is considered non-existent? I checked the other mods I downloaded and it seems that none of them use the version number format of x.x.0x

That's really strange, I have no issue using that download myself. And the version from here should be identical to the in-game mod browser. Do you have any mods installed that rely on mine by chance? It might be worth trying with just my mod enabled to see if there's still an issue, if that doesn't work please post your mod list so I can try myself.

I put the mod package downloaded from the webpage into a 2.0.28 game copy for testing. Even when only starting this mod, the same error occurred before startup. Then I tried downloading this mod in the game and it ran normally this time. Strangely, the version number displayed in the game was 2.1.3, but the mod package version number downloaded to the mod folder still displayed 2.1.03.
Anyway, it can still be used normally. Thank you

No worries, glad to help. I'm not sure what the issue is with that one download, but as long as it works now that's great.