Hi,
It seems like deactivation link was expired and this is WordPress issue and after this, you deleted the plugin via FTP and saw a common WordPress error that the plugin was lost.
If no – please describe all your steps, so that we could understand the problem. How did you try to deactivate the plugin and what error did you see.
Also, please specify, after you deleted the plugin on the “Plugins” page in WordPress admin dashboard, did you see this error – “constant errors that the plugin was lost”?
It’s a difficult question to answer, there are many unknown facts. Please provide the code of error from error log to this topic. Please read more about debugging log: https://codex.www.ads-software.com/Debugging_in_WordPress. A web server may have the error log.
Also, please provide your System status. Follow the next instruction – https://docs.google.com/document/d/1Wi2X8RdRGXk9kMszQy1xItJrpN0ncXgioH935MaBKtc.
Let us know about the results.