why is time machine not completing backups because "there is not enough space on the hard drive"? i thought time machine would automatically delete the oldest backups when the disk was running out of space?

You are correct however Time Machine is not perfect (what is?) and from time to time does not have enough space. The only thing to do is erase and start over or get a larger capacity External Hard Disk.

Here is what that message may mean:

1. Your startup drive is nearly out of space and Time Machine hasn't sufficient temporary space to perform the current backup.
2. Your Time Machine drive may be low on remaining storage, or the content that Time Machine would earmark for removal to make room for your backup is insufficient to meet that new backup storage requirement.

so just what is this "simple" solution???

It is interesting to me that this same issue seems to come up on this forum quite often and so far I have not seen a clear answer to the issue. My feeling is that Time Machine is an ok app, but it has its limitations. Such as the fact that it does not backup email messages. I much prefer Carbon Copy Cloner for a backup app. It is reliable, easy to use and has a lot of features not found in Time Machine.

Not true! First these forums are like a hospital so coming here to check the overall heath would give you false representations. I have used Time Machine for about 15 years, in those 15 years I had the above occur once! The solution was simple, overall Time Machine is extremely stable and reliable and like any app and I mean ANY app it can have quirks.