Thanks for reaching out in our Community forum and sharing your experience. Sorry to hear you've been dealing with this setback with your add to cart. I'm happy to help out.
To gather a little bit more context, can you please let me know the following:
In the meantime, if this persists further I will ask that you try to customize another theme in your admin. Does the same error occur? To do so, please follow these steps:
If you don't currently have any other themes downloaded, please download a free theme for testing purposes. To do so, head back into the 'Online store > Themes' section of your admin, click on 'Explore free themes', click on any free theme, and then click 'Add to theme library'. Once this theme has been downloaded, please follow the steps listed above. Once you are finished, feel free to delete the theme from your library.
I'm looking forward to hearing back from you so that we can continue to work on this together.
Thanks for getting back to me with that context! Do you remember which apps you added/deleted? Sometimes, app coding can remain in the theme even once deleted, for that reason it's important that we narrow down which apps you installed first. Did any of them have to do with the cart function?
It's showing up on my end. Did you experience it in one specific page? If so, which one?