Have you already tried with all other plugins disabled?
Viewing post in Summon System for RPG Maker MV & MZ comments
Here is what is going on with the plug in. The skill is corret. The note tags are correct. When I use the skill in battle is is bringing up a "Ghost skill" that does not exist. It's even using the wrong icon in battle as you can see in the last picture I do not have that book for my icon I have the bone knife as the icon. So its not even drawing the correct skill and I have no idea where it's pulling this mystery skill and icon from . It was working for 3 months straight until last night,
I have disabled all plug ins and it is still not working . 




The visu stella party plug in completely breaks this plug in. When I un check the box for the party plug in your MK summon plug in works without issue. When I enable the visu stella party plug in it totally disables your MK summon plug in . it does not work at all . The actors are not being summoned. The skill is used in battle and no actors appear. Also it shows the wrong skill icon, This all happens when I enable the visu stella party plug in . Which is disappointing because I need BOTH of these plugins,.
The problem for me is that visu stella party plug in wants the battle actors to be limited to 4. It will not allow me to have 5 actors in battle. I dont know why but it will not. And as soon as I disable it I can summon and I have 5 actors in battle LOL. So unfortunately I had to let it go . Thankfully theres other party plug ins