In this case you have:
confidence interval = 99%
sample size n = 27
In order to determine the critical t value you take into account the confidence level (99%) and degree of freedom. The degrees of freedom is calculated as follow:
degrees of freedom = n - 1 = 27 - 1 = 26
With this information, confidence level and degrees of freedom you search in a t table the point in which 26 and 99% cross each other.
The associated critical t value is t = 2.48, as you can notice in the following screenshot of a part of the table.
The corresponding t value is found in the row for 26, and there is the column for 99%
