No variables available for line-height property?

I can not connect the {line-height} property to a local variable {number} anymore.
It shows the message “no variables available”

All the existing styles with a line-height property in this file are connected to a variable, but if i want to change an existing one or create a new one - it shows this message

Is this a bug?
When i recreate in a new blanco file and just connect line-height to a variable it works.