In Power Automate when attempting to update an item I get the following error when the field allows multiple selections:

OpenApiOperationParameterValidationFailed.┬áThe ‘inputs.parameters’ of workflow operation ‘Update_item’ of type ‘OpenApiConnection’ is not valid. Error details: The API operation does not allow writing a value for parameter ‘item/itemName[0]/Id’. This parameter is read only.

Here are the steps to avoid this issue:

Near the top of your flow add a new step called Initialize Variable

type the name of what you want to variable to be called and select Array as the type. Leave Value blank

Next add a step called Append to array variable

For name enter the variable you used from above (e.g. RampstoInsert)

for value put in this code:

{
“Value”:
}

After the : insert the dynamic content of the multi value you are wanting. When you do that it will probably create an apply to each object which is normal

On your update item object click the T icon to switch to input array and put in your variable from above (e.g. RampstoInsert)

References:

https://wonderlaura.com/2020/09/01/flow-update-multi-select-column/