A trailing stop is a stop order that can be set at a defined percentage away from a security's current market price. An investor places a trailing stop for a long position below the security’s current market price; for a short position, they set it above the current price.
To set it on MT5, please see below instruction,
You should double check if the trailing stop points is checked after you set it.
The PC terminal is required to keep running to trailing stop works.