Can you meassure a negative pH value with a pH meter?
You are most likely getting an inaccurate value. As anticipated in Poutnik's comment above is difficult to reliably measure a $\mathrm{pH}$ of very acidic solution but the $\mathrm{pH}$ scale is indeed open, and negative $\mathrm{pH}$ are a real thing.
It is easy and more meaningful to state a very high concentration or activity rather than reporting inaccurate $\mathrm{pH}$. This, together with typical figures found in textbooks, has spread the misconception that $\mathrm{pH}$ is bound in the range 0 - 14, but it is not.
A simple example of a solution with negative $\mathrm{pH}$ is in every lab, namely conc. $\ce{HCl}$, 37% by mass, $\mathrm{pH} = -1.1$.
Edit: For sake of clarity, as for its definition, pH can exceed 14, too. As an example, a saturated NaOH aqueous solution has pH = 15.