Unhandled rejection SequelizeUniqueConstraintError: Validation error

The call to User.create() is returning a Promise.reject(), but there is no .catch(err) to handle it. Without catching the error and knowing the input values it's hard to say what the validation error is - the request.body.email could be too long, etc.

Catch the Promise reject to see the error/validation details

User.create({ name: request.body.email })
.then(function(user) {
    // you can now access the newly created user
    console.log('success', user.toJSON());
})
.catch(function(err) {
    // print the error details
    console.log(err, request.body.email);
});

Update, since it's 2019 and you can use async/await

try {
  const user = await User.create({ name: request.body.email });
  // you can now access the newly created user
  console.log('success', user.toJSON());
} catch (err) {
  // print the error details
  console.log(err, request.body.email);
}

I had this issue with my QA database. Sometimes a new record would save to the database, and sometimes it would fail. When performing the same process on my dev workstation it would succeed every time.

When I caught the error (per @doublesharp's good advice) and printed the full results to the console, it confirmed that a unique constraint as being violated - specifically, the primary key id column, which was set to default to an autoincremented value.

I had seeded my database with records, and even though the ids of those records were also set to autoincrement, the ids of the 200-some records were scattered between 1 and 2000, but the database's autoincrement sequence was set to start at 1. Usually the next id in sequence was unused, but occasionally it was already occupied, and the database would return this error.

I used the answer here to reset the sequence to start after the last of my seeded records, and now it works every time.

Check in your database if you have an Unique Constraint created, my guess is that you put some value to unique: true and changed it, but sequelize wasn't able to delete it's constraint from your database.