RTFM!
Your function “replace” in Javascript is only ONCE without global modifier.
See JavaScript String replace() Method
Example with global modifier in Kantu:
{
"Name": "test-replace",
"CreationDate": "2019-10-30",
"Commands": [
{
"Command": "store",
"Target": "Screen%20Shot%202019-10-28%20at%204.32.19%20PM.png",
"Value": "t"
},
{
"Command": "executeScript_Sandbox",
"Target": "var str = ${t}; return str.replace(/%20/g,\" \");",
"Value": "result"
},
{
"Command": "echo",
"Target": "${result}",
"Value": ""
}
]
}