Why is creating Foreign Key in Laravel 5.8 failing?

As we discussed in the comments above, a foreign key column must be the same data type as the primary key it references.

You declared your user.id primary key as $table->bigIncrements('id') which becomes BIGINT UNSIGNED AUTO_INCREMENT in MySQL syntax.

You must declare the foreign key as $table->unsignedBigInteger('user_id') which will become BIGINT UNSIGNED in MySQL, making it compatible with being a foreign key to the user.id column.