Let's figure out if this has something to do with your browser's cache. Sometimes, frequently accessed data stored in it may have caused this odd behavior. Let's perform a few troubleshooting steps to fix this.
We can start by opening a private window and try to add the tax payments once again. Here's how to open a private window:
Mozilla Firefox: Press CTRL + Shift + P
Safari: Press Command + Shift + N
Google Chrome: Press CTRL + Shift + N
If you're able to add them successfully, go back to your main browser and clear the cache. Otherwise, you can use other compatible browsers.
Here are tax-related articles for additional reference and guide: