Progress is an uncountable noun, and so:
▪ it never comes after a
▪ it is never used in the plural
✗ The 20th century has brought about a tremendous progress in science.
✓ The 20th century has brought about tremendous progress in science.
✗ During the last two centuries, technology, medicine, and psychology have made great progresses.
✓ During the last two centuries, technology, medicine, and psychology have made great progress.
Q. How can I refer to a single instance of progress, rather than to progress in general?
A. You can just use progress on its own:
They have made considerable progress.
You can also talk about an advance (advance is a countable noun) or, when referring to an action that improves a situation, a step forward:
Biochemists interested in nutrition laid the foundations for the next major advance in the treatment of cancer.
The Act was an important step forward in child protection.