It looks like you have Lijst met waarden selected on the metafield definition, so this will expect a list of single line strings.
You’ll need to create a new metafield definition if you only want a single value to be stored per key.
If you want multiple values to be stored for a single metafield key, then you’ll need to select list of single line strings as the type in Flow. Since the value is now a list of values type, your value field in Flow also needs to be a list, not just a single string.