There are many towers to choose from in Bloons Tower Defense 6, they each have different abilities and ranges and it is important to choose a tower that is beneficial to your game. The tower you choose must not be grey otherwise you do not have enough money to buy the tower, you can earn money by popping bloons.
In this example i will be placing a dart monkey.

Choose a place on the map that is close to the track to place your tower. The point you choose must be within range of the track and it best placed where it can see the most bloons on the track at a given time. It is also noted that placing a tower next to the spawning point of bloons is recommended as the tower will attack bloons early into the round. If the tower turns red then it is an invalid spot and you must choose elsewhere to place the tower.
In this case i will be placing my tower on the red circle.

You should have now placed your tower! Happy popping!

idk you failed the most basic thing in the game, good luck with that.