@Veli-Jussi Raitila Thanks for more clarification on the action.
I was able to reproduce the issue and got the exact error that you have mentioned if I am passing the incorrect primary key for my table.
In the action, please make sure you are passing the primary key value correctly as below. Using the same table schema that I have shared previously I was able to update the FName field in my table. Make sure that you are passing the primary key without double quotes or mentioning the column name etc. as it is not needed. If you want to update multiple rows then you need to pass the primary key values separated by comma.
You can click on advance parameters and then select the Primary key or the where condition as per your need.
In case if you are still facing the issue then please share the screenshot of the primary key that exists on the table along with your action value from the trigger history.