Why is 性 in the sentence 你不能吃凉性的水果?
“凉性水果”  doesn't  exist  in  Chinese  context.  “寒性水果”  is  the  right  way.

“寒性水果”  doesn't  mean  low  temperature  fruit,  or  low  cotent  of  cal.

It  comes  from  Chinese  “中医理论”  and  “阴阳论”。Four  catogries  can  be  summerised  as:  “性寒”、“性平”、“性温”  and  “性燥”。