Could you show your custom field rendering function? From one line alone it’s hard to say what’s going on exactly. I can only assume it’s the only line you have? In that case the error is expected: your function is supposed to return a UI Kit component, not returning anything will be an error.
For how to execute REST APIs in the rendering function, see here: Access the whole entity property object from a Forge Jira custom field rendering function - #3 by mskrzypkowski.