Account merging issue

Hi there,

I try to merge two Account records manually. System throwing error, the message is "The entity cannot be updated because it is read-only".

Record A has no order, opportunity and Quote opened/closed, but Record B has one opportunity closed, One Order closed and one quote open/closed.

Now I'm merging Record A into Record B or wise versa, now I'm getting above error. I've couple of plug-ins but they're not throwing any error.

Currently I'm using MSD CRM 2016 on-premise.

Any suggestions why this is not merging.

Thanks in advance.