Someone changed the tick rate with /tick
What it looks like in your log
Set the target tick rate to 10.0 per second
What causes it
The /tick rate command tells the server how many ticks to run every second. Above 20 the server is asked for more work than any machine can do, so every "Can't keep up" warning after that line is self-inflicted.
How to fix it
Run /tick rate 20 to put it back, and /tick unfreeze if someone froze it as well. If it was raised on purpose for a build session, expect the lag until it is reset.
Not sure this is your problem? Paste your log and Roastd will tell you which of these it actually matched, and what else it found.
Check your own log