When Bit Stuffing Is Used ,is It Possible For Loss ,insertion Or Modification Of A Single Bit To Cause An Error Not Detected By Checksum?If Not Why Not ?If So How?Does Checksum Play A Role There

It is possible. If data is 01111110 -> 011111010 if the second 0 is lost due to
transmission error, 01111110 will received, which can be interpreted as end of frame.
The bits before the end of the frame will then be interpreted as the check-sum. If the
check-sum is 4 bits, there is 1 chance in 24 that these random bits will be interpreted as a
valid check-sum. The longer the check-sum, the less likely that this mistake will happen.