It says, out of the blue, as it was previously working very well:
"

Error loading mods

Failed to load mods: SchallTankPlatoon/data.lua:6:SchallTankPlatoon/prototypes/ammo.lua:27:attempt to index field 'action' (a nil value)
stack traceback:
SchallTankPlatoon/prototypes/ammo.lua:27: in man chunk
[C]: in function 'require'
SchallTankPlatoon/data.lua:6: in main chunk
stack traceback:
[C] in function 'require'
SchallTankPlatoon/data.lua:6: in main chunk

What to do? The other day it was working fine.

What is your game version?
What is your mod version?
What other mods have you used?

Guess another mod(s) has modified some of the vanilla ammo. But I need to know which exactly, before figuring out how to fix.

Factorio is the latest version, fully up to date.
Tank Platoon is 0.17.14

Other mods that can have any effect on ammo:

Additionalturrets
Atomic Artillery
Robot Army
Vortik's Cannon Turret
Your artillery and alien tech mods

I'll try to mess around with disabling each of them individually to see whats up.

Yes, finding from your side is more direct and certainly very helpful.
You can do it like that.
Say the above ammo-related mods are named as A, B, C, D, E, F, etc.

Method 1:
1/ Disable all these (A to F). See if game starts well.
2/ If works, then enable A, and starts game.
3/ If works, then enable also B, and starts game.
4/ If works, then enable also C, and starts game.
5/ Repeat (one mod at a time) until you find the mod(s) that the game is not started well.

Method 2:
1/ Enable your full set of mods, except A. Then starts game.
2/ If it does not work yet, then disable also B, and starts game.
3/ If it does not work yet, then disable also C, and starts game.
4/ Repeat (one mod at a time) until you find the mod(s) that the game is started well.

These are the systematic way to find out what mods may have conflict.
After you find out the involved mod(s), I can start the fix.

Disabled some, including Additionalturrets, and it seemed to be the bad apple in the bunch. It is now working without a hitch. Thank your for the assistance, i didnt think about messing around with the active mods because all of them were up to date, but now that i think of it, it was foolish of me.
The issue is solved!

Thank you very much for your fast and very helpful replies, and also an incredibly huge thanks for making mods. I couldnt imagine my game without them! Thanks again!

Good to hear it is solved.
Right, it is getting harder to avoid mod conflicts when number of mods grow.
Maybe I will add some conditions checking next version to reduce conflicts, but still it may not work with everything.

Just released 0.17.15 / 0.18.3, which has added lots of conditions checking to reduce conflicts.
You may try re-enabling those mods again, but I cannot make any promise they would really work together.