Can I use USC 20 watts to charge IPad 6th gen

The repair tech is inexperienced. The iPad determines how much power comes into it, the power adapter's rating should be viewed as "max available power" it can deliver. So LotusPilot is correct, a 20W source is perfectly fine to use.

With comments of that nature, I’d be inclined to give that Repair Tech a very wide berth!

Anyone with even the least knowledge of electronics will know that, unless faulty, a device will only draw its maximum design power from a Power Supply. The Power Supply must be able to meet the power needs of a device, but a higher wattage supply will not cause damage. Having a higher design capacity will often result in lower temperatures - and in some cases, higher efficiency.

Whilst the Power Adapter must provide the appropriate output voltage (standard USB is 5V, whilst USB-PD can intelligently use higher voltages if requested/supported by the connected device), the power rating simply has to meet the minimum needs of the device. Higher rating allows faster charging up to the design limits of the connected device - beyond which you simply have “spare capacity”.

Yes, you can safely use the 20W USB-C Power Adapter with any iPad or iPhone.