That is calling a standard WordPress function. So what version of WordPress are you using? Is it out of date?

unfortunately we updated simple press  before updating  word press, but that seems to not mater as i cannot even get into my dashboard. 

1. Connect to your server using FTP.
2. Copy the simple-press folder in the WP plugins directory  to your local PC.
3. Delete the folder on the server.
4. Use WP and update WP to 4.7
5. FTP back and copy The simple-press folder back to plugins.
6. You will have to reactivate it.

As a general rule of thumb it is always a good idea to update WP first before any plugins…

copy which simple press folder ?

delete which folder on the server ?

how can I update WP when I cannot get  even get into my site or into my admin dashboard?

there is only 1 simple-press folder.. the plugin directory in the wp plugins folder… so wp-content/plugins/simple-press

ok did that all

site is working except the forum

And did you perform step 6?

yes

OK – so describing what ‘not working’ actually means might help. Some detail and information usually helps…

once I re-activated simple press the forum page no loner opens.

also this 

Post 11 means you have not updated all or any of the SP plugins you are using. In particular Private Messaging. Which is actually mentioned in the error message.

Post 12 eludes me. You said at the very beginning that you updated SP to 5.7.3. This message suggests this to be the case but at the same time it is saying that the SP files you have put back are NOT the newly updated ones.

they were the files in the wp/content sp folder.

Send me the sp-control.php file as an attachment. It is in the main ‘simple-press’ folder.