I don't know how samples_thin is created , nor what it does/should contain.
The error message is telling you that it can only plot objects with x and y components.
You have several options if you wish to receive further support. The most complete would be to provided a reproducible example : FAQ: How to do a minimal reproducible example ( reprex ) for beginners
Barring that, if you could share some of the code relating to the creation of samples_thin, that might give enough flavour to make some guesses.
Additionally, you can use
str() to print to console a string representation of samples_thin , maybe calling it on samples_thin[[1]] just to get the first member would be enough.
furthermore you could use the dput() function , on your objects, (mentioned in the reprex doc) as a way of sharing samples_thin with us. again we probably dont need all of it, just a portion.