1. If the start.bat file doesnt work try below methods.  
For Windows -> Add ↓ to a text file and save the text file as start.bat then run file.
java -Xmx4G -Xms4G -jar forge-1.15.2-31.1.25.jar nogui

For Mac -> Add ↓ to a text file and save the text file as start.command then run file.
#!/bin/bash
cd "$(dirname "$0")"
exec java -Xmx4G -Xms4G -jar forge-1.15.2-31.1.25.jar nogui

2. For the first startup, you have to agree to the Mojang EULA. Open eula.txt file, read EULA and change "false" to "true" if you accept it.

PS: Don't worry about the red cross before you connect to the server.