Your transaction log is full. Both Microsoft, and about 100 articles and blogs have covered this topic, but let’s take a quick look anyway. Because, you know, it comes up all the time.
This error message points to a lack of log backups.
Make sure using sys.databases.
Start backing up the log.
You can shrink the log if necessary.
A note on SIMPLE mode, and why it’s often a terrible idea.
This is a good summary of the problem and various solutions.