I got this message whenever I tried to fortify any troops and no troops moved, is this a rule in some games or a bug?

If it's a rule could you please explain it as I cannot find anything suggesting it is.

It means you had seige or navy and tried to use there movement for inf cav moves.
So if you had 4 cavalry 1 siege you would have 4 movables but not 4 movable cavalry.

Ok but I only had cavalry and infantry in the armies!

Basically, you need to keep at least one infantry or cavalry in a territory when you are fortifying. If you try to foritfy all the infantry and cavalry out, leaving only navy / siege / forts, then it will generate this error. Is this what occurred? If you left any infantry or cavalry behind, then this is a bug report

No I did try to move my entire available army but it dident allow me to move any, I'll take the next turm and experiment with it and get back to you.

Okay. When you try to move but it disallows the move (via a popup), then you shouldn't lose any movement points. It's basically the game engine saying "Hey wait a minute, I'm cancelling that action because it's not a legal move". If you lost your movement point, then again, that's a bug. Here's the deal:

If you try to move all sentient troops, the engine should throw an error (a popup screen)
No movement points should be used
No troops should move

If something other than that happens, then it's a bug in the program, which I'll fix right away. From what you've described, it's difficult to determine whether it's a problem in the program, or if it's just a misunderstanding of the rules (which really is my fault because I have to write better instructions)

Pretty sure the error was Me, that said would it not be better for the game to calculate that a sentient unit needs to remain and adjust the numbers accordingly ie:- if their are 4 cavalry and 4 infantry in a location and a player is using the fortify box to move them if they click to move the 4 infantry then the box automatically adjusts the available cavalry to 3 or vice versa.

I'm only saying this as this sort of confusion can turn away casual players and it can be easily avoided.

Yeah that would be good, the algorithm will require a bit of brainwork though. I'll look into it