Integer overflow detected

[snaggle]

first off, why are you using --force? This can cause issues cracking. Hashcat gives you reasons as to solve why it is not working correctly and you should listen. 
secondly, your mask is completely wrong. See the wiki for help creating your mask. You're attempting a 20 character password which is way to big which is causing your integer overflow.

https://hashcat.net/wiki/doku.php?id=mask_attack

Thanks for the response.
First thanks for the tip, I was using--gorce to literally try and force it!
I've spent a long time with the wiki before asking here, obviously I'm struggling or else I wouldn't of asked for help.
Any more granular help would be greatly appreciated.