You missed return in execute script try in this mode
{
"Name": "LoverCase",
"CreationDate": "2020-11-29",
"Commands": [
{
"Command": "store",
"Target": "AAA",
"Value": "Var"
},
{
"Command": "executeScript_Sandbox",
"Target": "return ${Var}.toLowerCase()",
"Value": "Var"
}
]
}