Were you using the tool after loading in a save?

No.
To elaborate further, I uninstalled the addon, cleared the old files, installed it again, had the same problem.
Everytime I tried on a model I would close Gmod and open it again

I have same issue

usually this error happens if the gizmo entity didn't spawn in, selecting the tool or switching to it should spawn one if there wasn't one already. can you try to check if that still happens if you disable all addons except for ragdoll mover? also what the hell steam didn't subscribe me to this discussion even though i did reply. so pardon for late response.

It works with all the addons turned off, I checked.
But I do not know which addon can conflict with it

oh boy. you'd have to do a thing with checking that, if you have time. usually what people do, is enable page of addons, load a map, see if the problem arises. once it happens, you can start narrowing down the addons from the last enabled page, until you leave in just ragdoll mover and other addon, and that would be the conflicting thing. hope that makes sense.

Alright, I think I found out the problem (but I could be wrong): basically, since the tool was previous selected, it was automatically chosen when switching to it upon creating a new game/session but I'm assuming the lua code didn't kick in because it wasn't "selected".
Also, it seems to have worked with all my addons enabled and disabled, so not externally.
For those who have this problem, select a standard Garry's Mod tool when first loaded into the map, THEN select the ragdoll mover. That did it for me

yeah, that's the thing with saves if you had the ragmover out. saves essentially wipe the map, ragdoll mover's gizmo entity also gets yeeted. it only spawns new one when deploy function is ran, as it checks if gizmo entity exists or not and then spawns a new one if none is detected. technically switching from ragmover to another tool, or from toolgun to weapon, like physgun, back to toolgun should run the deploy function.

Now it working! Just need to switch to another tool

I used to have the same issue about 3-4 years ago, just uninstall and reinstall maybe? i uninstalled for like 3 years and re-installed and then it worked.