• Once you find ptr1, you set preptr and ptr2 to ptr1.
• Then you get c = ptr2->data, which happens to be the same as ptr1->data.
• And you go on and check while (ptr1->data != c), which is always false. So that bottom while loop does nothing
• And you get to the last lines with preptr and ptr1 pointing to the same node n.
• Now you make n->next (preptr->next) point to newnode, and newnode->next point to ptr1 (n): an infinite loop.

               n           newnode
            --------      ---------
preptr ---> |  c   |      | value |
ptr1   ---> --------      ---------
            | next | ---> |       |
            |      | <--- |  next |
            --------      ---------

void insertt(struct node *head) {
    struct node *ptr1 = head;
    int pos, poss, value;
    cin >> poss;
    cin >> value;
    pos = poss - 1;
    while (pos--) {
        ptr1 = ptr1->next;
    }
    struct node *ptr2 = ptr1;
    struct node *newnode = (struct node *) malloc(sizeof(struct node));
    newnode->next = NULL;
    newnode->data = value;
    struct node *preptr;
    preptr = ptr1;  // ptr1, ptr2, and preptr point to the same node n
    int c = ptr2->data;  // c = ptr1->data too
    while (ptr1->data != c) {  // always false
        preptr = ptr1;
        ptr1 = ptr1->next;
    }
    preptr->next = newnode;  // n->next = newnode
    newnode->next = ptr1;  // newnode-next = n

    display(head);
}

Here is a possible solution. to be honest your code was a bit messy and it was easier to just rewrite it. I am not sure what is the problem with your but you could compare with the code below and try to figure it out.
A couple of notes:
You do not need to have *ptr and *head as global variables it is better to keep them in the main.
The way you navigate over the list with the while loop to get to location is a bit unclear and you can do it with a for loop in a more readable way (in my opinion).
There is no error prevention/handling which makes me very nervous but if this is just for toying with, you should be fine

// Example program
#include <iostream>
#include<stdlib.h>
using namespace std;

struct node{
    int data;
    struct node *next;
};

void insertt( node *head, int pos, int val){   //insert function to insert node before a node
    struct node *ptr = head;
    struct node *next;
    
    for (int i = 0; i < pos-1; i++)
        ptr=ptr->next;
    
    next = ptr->next;
    
    ptr->next =  (struct node*)malloc(sizeof(struct node)); //creating new node for insertion
    ptr->next->data = val;
    ptr->next->next = next;
      
}

void rdisplay(struct node *head){  //displaying linked list
    struct node *ptr=head;
    if(ptr!=NULL){
        if (ptr->next!=NULL)
            rdisplay(ptr->next);
        cout<<ptr->data <<std::endl;        
    }
}

void fdisplay(struct node *head){  //displaying linked list
    struct node *ptr=head;
    if(ptr!=NULL){
        std::cout << ptr->data << std::endl; 
        if (ptr->next!=NULL)
            fdisplay(ptr->next);              
    }
}

int main()
{
    struct node *ptr = NULL,*head=NULL;
    int n,val,i;
    std::cout << "Incert number of values to have in the list:";
    cin>>n;   //number of nodes
    for(i=0;i<n;i++){        //node creation
        std::cout << "Incert value for node[" << i << "]:";
        cin>>val;
        struct node *newnode = (struct node*)malloc(sizeof(struct node));
        newnode->data=val;
        newnode->next=NULL;
        if(head==NULL){
            head=newnode;
        }
        else{
            ptr=head;
            while(ptr->next!=NULL){
                ptr=ptr->next;
            }
            ptr->next=newnode;
        }
    }
    std::cout <<std::endl;
    std::cout <<std::endl;
    fdisplay(head);
    std::cout <<std::endl;
    std::cout <<std::endl;
    int pos;
    std::cout << "At which position do you want to incert: ";
    cin>>pos;   //getting position and value
    std::cout << "What value do you want to incert: ";
    cin>>val;
    insertt(head, pos, val); //insertion
    std::cout <<std::endl;
    std::cout <<std::endl;
    fdisplay(head);
    std::cout <<std::endl;
    std::cout <<std::endl;
    return 0;
}