Hey @Carlotta1 .
Thanks for reaching out.
I appreciate that you have included an image of the error message as that is helpful. Did you happen to make any coding changes to your theme? If so, we can roll back the themes coding to revert any changes that have been made. It looks like this error may be coming from the themes cart.liquid section of the code.
You can make your way to the themes coding by navigating to Online store > Actions > Edit Coding. You should notice that if there has been changes to any of the coding templates there will be a purple dot next to the name of the section. I took a screenshot from my test store of the area I am referring to:
Once you select on the cart.liquid section, you can easily revert the changes made to the cart back to its original coding.
Let me know if that does the trick, looking forward to your reply.