This is an important question, especially in contexts where access to and affordability of soap is limited. Unfortunately, there is very little robust research on ash, let alone specifically in the context of Covid-19.
Ash lowers the ph-value of the skin, creating an unfavourable environment for pathogen. Yet, there is no evidence available to date that it is effective against the novel coronavirus. If ash is used, it should be taken from a clean environment free of human or animal feces as well as heavy metals. In sum, where no water and soap are available, ash can be used as a last resort.
Have you had experience with using ash for handwashing? Please share it below!
Official guidance by UNICEF and WHO can be found here: https://www.who.int/publications/i/item/water-sanitation-hygiene-and-waste-management-for-the-covid-19-virus-interim-guidance13
A more detailed answer can be found here: https://resources.hygienehub.info/en/articles/3915810-can-ash-be-used-for-handwashing22