You are nearing the limit of your local storage.
Your browser only permits you to store a certain amount of data locally per domain, and you are nearly using it all.
For most modern browsers, that limit is 5MB, or around 250,000 characters!
There are a few ways you can deal with this.
1. Save files on your computer. You can press the download icon next to the file name to save a file onto your own machine, and you can press the upload button to upload it back again later.
2. Delete unneccesary files or boxes. You can delete a file, if you have more than one file, by clicking the bin icon next to its name at the top of your screen.
You can delete a box by clicking on it until the toolbox comes up, and clicking the big red delete button.
3. Switch browser. (E.g, if you're using Chrome, switch to Edge, or Firefox.) If you use a different browser, you get a new 5MB limit! And that should be enough for all the WorkFlow files you'll ever need. 10MB is enough for 100 files with 100 boxes, each with 100 characters!
4. I don't recommend this, but if you want to clear all your storage, and I really mean everything, you can click on the storage widget.