So I installed this plugin, on Valheimplus 0.9.9
as soon as I added it I was unable to join with version mismatch error.

I removed the plugin it didn't help, 
I had to completely reinstall the valheim plus server in order to join it again.

anyone else able to replicate this issue?

This plugin does nothing that would cause that. Sounds like you have an older version of uMod that you are installing on top of Valheim+.