Sharepoint - Field Modified shows date, but no time

The point is that formula expression is usually language dependent.

For example, is site locate is set to 1033 (English - United States), then the formula you have provided in the question is absolutely correct:

Syntax: TEXT(value,format_text) Source

But let's say the site locate is set to 1049 (Russia), in that case the proper syntax would be:

ТЕКСТ(значение;формат)

What matters here is delimiter, as you can see it is ; for 1049 locale and not , like in case of 1033 locale.

Solution

Identify the locate of SharePoint site (go to Regional Settings - see the figure 1)
Go to A list of available functions and verify the proper syntax for a function according to the language specified (see figure 2 and 3)

Figure 1. How to identify Locale settings enter image description here

Figure 2. TEXT formula for 1049 locale (e.g. 1049 locale) enter image description here

Figure 3 How to determine formula syntax

enter image description here

Update

How to set Time Format enter image description here

References