Why it's not printing the string I inpuutted ?

Try

#include <stdio.h>
#include <string.h>
#include <stdlib.h>

#define CHARS 30

int main()
{
  int c, categories;
  printf("how many types/categories of meds you want?\n");
  scanf(" %d", &categories);
  printf(" %d", categories);
  char * category[categories];

  printf("\nPlease mention the categories of meds like B.P, anxiety, blood thinners,etc:\n\n");



  for( c = 0; c < categories; ++c)
  {
    unsigned long len = 0;
    category[c]= malloc(CHARS * sizeof(char));
    printf("CATEGORY #%d ?\n\n", (c+1));

    while (len < 2)
    {
      fgets(category[c], CHARS, stdin);
      len = strlen(category[c]);
    }
  }

  for ( c = 0; c < categories ; ++c)
    printf(" %s", category[c]);

  for ( c = 0; c < categories; ++c)
    free(category[c]);

  return 0;
}

You have two problems:
1 the use of scanf to read values will leave a line end in the input buffer, which causes the next input command to read the wrong data. You need to clear the buffer after the call to scanf:

char dummy[4];
gets(dummy);

2. You are missing the offset for the buffer address when you read the medicine details:

gets(category); // should be          gets(category[i]);

As Richard has already correctly pointed out, this line does not work:

gets(category);

Here you want to read in several lines. But the command obviously always overwrites the same place, and one that is not intended for that.

The gets() function has since been removed from C and C++ for good reason. There are almost the same problems as with scanf() with %s, since the string length cannot be limited. An alternative that reads only to the end of the line and at the same time respects the buffer size does not exist under C. If it absolutely must be C it becomes somewhat more complex, since one must write the desired function.

As said before, a function to read in a line with a limited buffer would have to be written by yourself if necessary:

int mygetline(char *buf, size_t len)
{
	char ch;
	for (int k = 0; (k <= len); ) {
		ch = getchar();
		if ((ch != '\r') && (ch != '\n')) {
			buf[k] = ch;
			k++;
		}
		else if (k>0) break;
	}
	if (ch != '\n') {
		while (ch = getchar() != '\n');  // delete rest of line
		return 1;
	}
	else
		return 0;
}

Note: With C++ you could use getline() as an alternative.

When inserted into the (corrected) original program, it looks like this:

printf("Please mention the %d categories:\n", types);
for (i=0; i<types; i++) {
	category[i] = (char*)calloc(types, (MAX_LINELEN + 1) * sizeof(char));
	if (category[i] == NULL) 
		break;

	printf("CATEGORY #%d:\n", i + 1);
	mygetline(category[i], MAX_LINELEN);
}