I am unable to autotune on 21Mhz as the SWR is more than 1:3 which is out side the range of the radio. I would like use it only for digital TX with a max. power of 50 watts. Any solution/s?
The traditional "No balun" G5RV design is famous (infamous, more like) for these sorts of quirks, primarily because the claim that it behaves in various different (and universally desirable) ways at different frequencies is a crock. In reality, it has two modes, a balanced dipole when used on a frequency where SWR (and in theory, common mode current) is low (30 and 17m on the Lite), and a sort of Frankenstein vertical (driven by common mode) and dipole (driven by the differential mode) everywhere else. That second mode is unpredictable at best, because the coax shield is just hanging there begging to act as part of the antenna, and the ladderline likes to radiate to varying degrees depending on frequency and severity of imbalance.
In your case, a quick model suggests you've got several hundred ohms of inductive reactance at the junction between the coax and ladderline at 21MHz, which may be more than your tuner can handle.
The best way to handle this would be to extend the ladder line to the shack window, or as close as you can get it, place a good quality 2 core 4:1 current balun there, and then run a short length of low loss coax like RG-213 in to the tuner. This should drastically reduce your feedline losses, and generally results in a much broader useful bandwidth overall. In your case, if this model is even remotely close to accurate, you're likely losing more than 50% of your power in the coax run, even if you're using just 50 feet of RG-8X in the current setup.
The "band aid" solution would be to place a bit of capacitive reactance across the ladderline to counteract the inductive reactance. I'm not sure exactly how much capacitance you might need, as that's going to be highly situation dependent. I would recommend using a variable capacitor at low power to determine how much is necessary to get below 4:1 or so, and then swap the variable capacitor out for a fixed capacitor with at least a 1kV voltage rating when you want to run 15 meters, and remove it otherwise.
A final (and maybe optimal) solution, but requiring substantial financial investment, would be to place a remote tuner like an Icom AH-4 or LDG RT-100 at the point where the ladderline from the G5RV ends, and run coax to the tuner. This is probably the best of all worlds, keeping coax loss low, giving maximum bandwidth, making it easy to isolate the coax from the antenna, and eliminating the frustrations of running ladderline long distances.
An off centre fed dipole is a better bet if you want multi band operation. Just use a good dual core Guanella current balun and a line isolator choke and you'll have no problems.
Don't add half a wavelength of coax. Add a quarter wavelength (at 15m) ideally take the velocity factor of the coax into account.
As Hamsterdave suggests, you could use a different antenna that has a lower SWR on 15m, or a remote antenna tuner. You could also use an external antenna tuner in the shack, which would be lossier but perhaps more versatile. You'd want one that can be entirely bypassed, because antenna tuners are inherently lossy, and you want to bypass it if you don't need it. Automatic antenna tuners are certainly popular now, but manual tuners cost a lot less, and they aren't difficult to use.
High SWR is bad for transmitters because it creates higher voltages or temperatures at the final amplifier stage. If you reduce the output power, that of course reduces the voltage and temperature of the final amplifier stage, so your rig may be able to tolerate a higher SWR mismatch if you turn down the power, so you may not have a problem at all at 50 W.
The usual solution to autotuner problems is to change the length of the coax.
Try adding a half wavelength of coax, approx 5 m, to the antenna feed, and see if that helps. You could also try 2.5 or 7.5 m, anything about that long will have some impact.
Although the VSWR won't change much if you add a bit of cable, the impedance will change completely, from inductive to capacitive, from high to low, or vice versa. You will likely move from a spot where the tuner can't cope, to a better spot where it can match the antenna.
While you are at it, do you at least have a coil of coax or some other choke at the base of the G5RV? If not, it might help all bands to have 5 turns about 20-30 cm in diameter, right at the end of the ladder line.