Problem with the namespaces declaration in the relations (upper case). You need to change your relations to:
// Portfolio class
public function portfoliotxt()
{
return $this->hasMany(PortfolioText::class); // or $this->hasMany('App\PortfolioText');
}
// PortfolioText class
public function portfolio()
{
// Your relation type must be 'Belongs to', not 'Has one'
return $this->belongsTo(Portfolio::class); // or $this->belongsTo('App\Portfolio');
}