The key may be from a Volume License (school/business) that requires a special installer, or it could be a counterfeit key. Solution: Contact the reseller you bought from. If you bought from Adobe directly, contact Adobe Support. Never download "patched" installers from third-party sites.
Most perpetual licenses allow 2 activations per computer. You may have previously installed it on another PC or reinstalled Windows too many times. Solution: how to install adobe acrobat pro dc with license key work
By using a legitimate license key and following this guide, you will have a stable, secure, fully-functional copy of Adobe Acrobat Pro DC that respects your ownership and protects your data. The key may be from a Volume License