Need help in SP2016 migration.

Question

I am migrating SP2013 to SP2016 using content db mounting. All the custom solution I have deployed to SP2016. But when I am running test mount command before mounting.. getting lot of error one them is "site definition" is not found in target web application.. My question is since I have deployed all custom solutions why I am still getting this error. Or if I leave this error as is and go ahead with the mount content db command, will it be any issue? Other error in test content db is feature mismatch, setup of files not found... if I ignore this error and start with the mount command what will be the issue or this is fine??? Please advise experts, based on your input I will continue.