The water level in toilet cistern is too high. Look for the following things:
If the float valve is passing there will be a constant drip on the valve when you take the lid off the toilet cistern. This can be solved by replacing the valve or the washer kit for the valve. If its a standard valve I personally change the whole valve.
If the float has split and filled with water it won’t float so won’t close the valve fully. On older valves the ball unscrews and has a standard thread so a new one screws back on.
If the water level has been set too high the valve will never shut and will constantly overfill. The level can be adjusted by bending the lever arm of the ball valve. Or on other valves there are adjustment screws to set the water level.
There are many different types of flush valve for toilets. All have different dimensions and it can be very difficult to match the right flush device to your toilet. These are some examples available from amazon:
Some more modern toilets have overflows that go into the toilet pan. This avoids the need for a pipe to be run outside. The valves that go in these toilets have to be the right size to fit the cistern. It is best to contact the manufacturer as there is no standard one size fits all solution.