Cannot find the startup method defined in default.json

Check your Default.json file, it should have this code:
{
"url": "Default.html",
"startup": "WisejWebPageApplication1.Program.Main, WisejWebPageApplication1"
}

“WisejWebPageApplication1” will be different for your project.

For example if your namespace was NAMESPACE1 :
namespace NAMESPACE1 {
internal static class Program {
//more code..

and your assembly name was ASSEMBLYNAME1:
(Note: By default the assembly name will match the project name, but you can override it in the .csproj file with the “AssemblyName” tag)

then your default.json would look like this:
{
"url": "Default.html",
"startup": "NAMESPACE1.Program.Main, ASSEMBLYNAME1"
}

If your default.json file looks fine, check to make sure that there’s not a permissions issue with accessing/running the startup file.