iTunes wants to erase my iPad

I have an iPad Air 1 (iOS 902), MacBook Pro (1 month old, El Capitan).

Due to syncing problems I tried to change some settings. But when I do this, iTunes asks me to erase my iPad.

Details:

I sync with USB (iPad - Macbook)

When I make a copy, my new apps and updated apps at iPad are not synced with iTunes

In iTunes I also see my latest downloaded apps at iPad are not in my iTunes Library.

Starting syncing, or making copy does't give error message.

When I try changing preferences like: sync with Wifi, or make local copy.

Then I get an error message: can't sync library because iPad is linked to another library at old laptop.

This laptop I do not longer have (formatted),

And iTunes ask me to erase iPad and use library of MacBook. But I can't do this because the library at my Mac is not ok: old apps , missing apps.

I had no syncing / copy problems until 2 weeks ago (update iOS)

1. How do I get my apps synced with iTunes (usb) - Mac
2. How can iTunes again download apps, I already installed at my iPad
3. How to solve the library issues