The players in my server cant seem to locate or mistakenly enter the portal please help me to solve this problem also one user said type [ /mvpm fill ] command would work but it didn't please help.

You can place water or lava in a portal to make it show like below

So now I'm guessing you are using a modded server. We don't support modded servers. Only spigot/paper etc.
So as Zax said.... Try it and see.

Modded servers are not supported

Wont the water/lava flow out?

https://tryitands.ee nope