Like it says, you can’t do maths on an Item state that is NULL.
All openHAB Items start life with state NULL.
Something has to initialise them - binding reading a value from a device, perhaps.
Or, you might write your rule to deal with the NULL case by making an assumption about the start value.
@rossko57: do you can give me an example please? I’m a absolute beginner, I work with Openhab since October.
This might help