I think OP does not want to do the whole sync process from the beginning but I don't think OP has any choice because the re-index will probably restart a new sync process which OP is trying to avoid in the first place ...
Where do you read from OP's post that he tries to avoid a new sync or re-index?
OP clearly states he's not tech-savvy and likely lacks some Bitcoin Core knowledge and thus he doesn't understand what the error message tells him and implications from that.
We can't know for sure why his node is pruned, might just be the default setting and he didn't know about potential consequences, especially when you introduce foreign wallet.dat file to such a pruned node. Could also be that his device lacks the necessary amount of storage to hold an unpruned blockchain.
OP can't avoid a new sync of all blockchain's blocks for his pruned node to properly sync his foreign wallet.dat file.