my Time Machine message is that its full and not enough room to perform backup. what suggestion can you make to free up space on Time Machine?

You will either have to reformat it and start over or you should consider getting a larger capacity EHD. If you go for another EHD, I (and many other experienced users) prefer the OWC Mercury Elite Pro which can be purchased in a variety of capacities. You can find them at:

https://eshop.macsales.com/shop/owc-mercury-elite-pro

In addition, you can transfer the data on the current TM drive to the new one. Apple explains how to do this in https://support.apple.com/en-us/HT202380

How much free space do you have on your boot drive? If it doesn't have enough room for a local snapshot TM can't backup. Make sure you have at least 20 GB of free space available, more if possible.

TM will automatically delete the oldest backups when it need additional space for the next backup.

Spike45 - TimeMachine should delete older TM backups to allow new ones but there are circumstances when this can’t happen. Usually this occurs when your TM hard drive isn’t large enough. Most of us recommend a TM drive that is 2-3 times larger than your backup source. As Old Toad suggested, it might also be a damaged HD.

I really hate erasing a TM drive when it is the only backup available. Murphy’s Law dictates that this is the perfect time for your computer to fail. My best suggestion is to look at how much stuff you have - multiply that by 3 - and look for a high quality drive that is that large or larger. In other words, if your drive currently has 400GB of stuff on it, your minimum backup drive should be 1TB. Then start using it as your new backup drive. After a couple weeks you erase the old drive or keep it off to the side. for what it is worth I like to keep two separate TM backups. Belts and suspenders.

Old Toad wrote:

TM will automatically delete the oldest backups when it need additional space for the next backup.

FWIW, there is a limit. As rkaufmann87 noted, It is possible Time Machine can no longer make enough space by deleting the oldest backups and will require a larger drive.

Not unless the drive is damaged and, in that case, will need to be erased and start over again.

Also I find it unlikely unless the size of the driver can hold only a couple of backups. The only time I've had a problem was when my wife's iMac got down to less than 5 GB and didn't have enough space on the boot drive for a new local snapshot.

The OP can run Disk Utility's First Aid on the TM drive and then give it another go. The First Aid run will take a long time due to the complex ecosystem of file permissions and hard links.