I tested this in my install.json and it worked fine
"resources": {
"body": [
{
"type": "style",
"src": "https://stackpath.bootstrapcdn.com/bootstrap/4.1.1/css/bootstrap.min.css"
},
{
"type": "style",
"src": "./source/app.css"
},
{
"type": "script",
"src": "./source/app.js"
}
]
},
"preview": {
"handlers": [
{
"options": ["_default"],
"execute": "INSTALL_SCOPE.setOptions(INSTALL_OPTIONS)"
},
{
"options": ["_product"],
"execute": "INSTALL_SCOPE.setProduct(INSTALL_PRODUCT)"
}
]
},
"options": {}
}
There is likely something wrong with the JSON of your install.json. You could try copy and paste your install.json in this tool to test: https://www.cloudflare.com/apps/developer/install-json-tester