Excelsior

why can't i fix it?

It looks like you're mounted?

You can't repair items while you're mounted, I think.

thanks that too but I need leather to fix it

take Sewing Kit
select preparations
click on Field Repair Kit - select leather (Normal)
and make it

later on the road click on Field Repair Kit, point to leather item, it will be repaired , taking some uses from the kit.

Most convenient way.
If you have items from other types of leather make field repair kit from that type of leather.

According to the gump, you have 0 of the 17 leather needed to repair it in your backpack.