Here is the code. Someone told me system.run was what to use. Here is my script.
if (result.selection === 1) {
modal6.show(player).then(result => {
if (result.cancelled) return;
var pname = getPlayerNames()[result.formValues[0]]
player.runCommandAsync(`execute as @p[name = ${pname}] at @s run fill ~~~ ~~~ powder_snow`);
player.runCommandAsync(`effect ${pname} slowness ${result.formValues[1]} 255 true`);
system.run(`execute as @p[name=${pname} at @s run fil ~~~ ~~~ air`) => {
result.formValues[1] * 20
};