Hi Katelyn,
There are several possible reasons for this error message.
1. The most probable is that you are using older version of CLANc. Try updating to the latest version.
2. The second reason is that the folder that MLU is trying to write the "sample.mlu.cex" file to is write protected. Try copying the sample.cha file to Desktop and run MLU there.
3. The last, and least likely, reason would depend on the MLU command line you are using and the "sample.mlu.cex" is opened by some other editor that locks that file against writing to it.
If you are still having this problem, then try rebooting your Mac. There is another possible reason that has to do with Ventura's security protocol, but it is very unlikely. Please let me know you if in the end you are still having problems.