Is a cumber a fruit?

A cucumber is classed as a fruit because it forms at the base of a flower, just like an apple, a pear, tomato or orange.

Because a fruit holds seeds. It "fruits" (protects) them until the fruit's exterior has been used in some sort of way, and, the seeds are scattered.

If you mean why does " your fruit tree have no fruit " than it's either the tree requires another fruit tree to pollinate it or trees take time to mature before bearing fruits.

yes

It is a simple fruit - botanically a berry.