Short answer. Not on that ground. Article 281 allows termination for failure to qualify only against reasonable standards made known to the employee at the time of engagement. Standards never communicated, or communicated only when the evaluation came round, cannot support a finding that you failed them.
What the law says
when he fails to qualify as a regular employee in accordance with reasonable standards made known by the employer to the employee at the time of his engagement
Labor Code, Article 281 — Probationary Employment. Read the full provision →
Three conditions, all in one clause
The article permits termination when he fails to qualify as a regular employee in accordance with reasonable standards made known by the employer to the employee at the time of his engagement. Pull the clause apart and it asks three things of the employer. The standards must be reasonable. They must have been made known to you. And they must have been made known at the time of engagement — at the start, not midway, and not in the memorandum that ends the employment. Failing any one of the three, the ground is unavailable however poor the employer says your work was.
What counts as making standards known
The requirement is one of communication, so it is met by telling you what you would be measured on, in terms you could actually work to. A contract clause reserving the employer's sole discretion to assess you communicates nothing. Neither does a general exhortation to perform well, nor a policy document you were never given. Standards conveyed verbally can qualify — the article does not demand a form — but an employer relying on a conversation has the awkward task of proving what was said and when.
The other ground is still open to the employer
Be clear about what this does and does not settle. Article 281 gives two ways to end a probationary employment: failure to qualify against known standards, and a just cause. Those are separate. An employer with no standards to point to may still proceed on a just cause under the Code's list, and it then has to prove the conduct it alleges like any other employer. What it cannot do is dress up a general dissatisfaction as a failed evaluation when there was never anything to evaluate against.
What to look for in your papers
Go back to what you were given on or around your first day: the appointment letter, the job description, any orientation material, evaluation forms, and emails setting targets. Note the dates on each — the question is what existed at engagement, so a form dated in your fifth month is telling. Keep the termination memorandum too, since the reason it states is the one the employer must stand behind. And check the calendar, because an employee allowed to work past six months is regular regardless of any evaluation.