Lightning Forms has a great many features and capabilities, and many errors you may see will come from SharePoint itself. In this article, we will discuss some of the more common errors you might see when developing and using Lightning Forms customizations.
The field you are trying to update may be read only.
You may see this error when using the Add list item or Update list item action. The error message may also say: “Invalid data has been used to update the list item.” This most likely means that you have entered a placeholder or expression in the action's field, that's of a different data type than the field in the target list. A common example is that if you are trying to copy a Lookup value, rather than specify its placeholder (for example, [[Department]]), you will need to specify its Key, such as [[Department.Key]]. This is because SharePoint stores Lookup fields as key/value pairs (e.g. 1:Sales, 2:Accounting).