Its the VISU STELLA party system!! IT has a "max limit" party member parameter and I had it set to 4 max members!
Viewing post in Summon System for RPG Maker MV & MZ comments
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,.
When my plugin works with CGMZ but not with VS, it is possible that the problem lies more in VS's hands than in mine.
Temporarily bypassing the party limit (i.e. 5 actors when the limit is 4) is not well supported in RPG Maker. Yes, there is the "exceed limit?" option in MK_Summon, but I no longer work on it. Also see its description on my blog:

https://decinbr.itch.io/rpg-maker-mz-plugin-party-swap-css-style
This plug in works with your MK summon and it has a party scene like visu stella. I would recommend this to others having the same problem as myself. It p robably just saved my game. I can now use both plug ins and return to my work. Thanks.
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