Check the device’s date & time.
If the certificate issued is before the date and time, the application cannot be installed as the certificate is considered invalid.
Check if you have done the Permit to install applications step.

No, if the author key is different, any application cannot be upgraded.

No, an app cannot be updated if you do not have its author certificate. When updating an app in Seller Portal, the author certificate of the original binary and its updated binary must match. If the certificates do not match, you cannot register the updated binary in Seller Portal.

If you are only going to use the emulator or if you are only going to distribute your app in Galaxy Store, you only need one (primary) distributor certificate.

If you are going to distribute your app in another app store, you can provide a secondary distributor certificate for that app store.

In your certificate profile, you can have up to two distributor certificates:

• The primary distributor certificate is automatically generated for you when you create your certificate profile (this certificate is also referred to as a personal distributor certificate). The personal distributor certificate is for development purposes only. When you publish your app to Galaxy Store, the primary certificate is replaced by Galaxy Store’s distributor certificate.

• The secondary distributor certificate is optional and must be a valid non-personal distributor certificate. The secondary distributor certificate is typically the distributor certificate of another app store. When you publish your app to Galaxy Store, the secondary certificate is retained.
  Do not add a secondary personal distributor certificate. If you add a secondary personal distributor certificate to your certificate profile, it is not replaced when the app is published to Galaxy Store. This certificate will be validated when the app is installed on a device. If this certificate is not valid (for example, it has expired), the app will not install on the device.
  If you have added a secondary distributor certificate that is a personal distributor certificate, remove it from your certificate profile.

Note :

Seller Portal validates the primary distributor certificate only. A device validates all personal distributor certificates.

No, using the Certificate Manager in Tizen Studio, you cannot reset a certificate's password. If you forget the password for your author certificate, you must regenerate a new author certificate. You will not be able to publish updates to any app published with the original author certificate. If you want to publish updates to your apps that used the original author certificate, you must republish the original apps using the regenerated author certificate.

There are two types of distributor certificates, primary and secondary, that can be added to your certificate profile. What happens depends on which distributor certificate has expired. See Q04: Do I need more than one distributor certificate? for more information about the types of distributor certificates.

Primary distributor certificate: personal
A primary personal distributor certificate is used for testing only. When you release your app for sale in Galaxy Store, your primary personal distributor certificate is replaced by an official distributor certificate.

If your primary personal distributor certificate has expired, you cannot install your app from Tizen Studio to your watch until you get a new certificate. However, the expired certificate does not affect any apps that have already been installed on your watch nor any apps that are for sale on or have been sold from Galaxy Store.

Secondary distributor certificate: personal
If you have a secondary personal distributor certificate, remove it from your certificate profile.

Secondary distributor certificate: non-personal
A secondary non-personal distributor certificate may be added to your certificate profile and it is retained when your app is released for sale in Galaxy Store.

If your secondary non-personal distributor certificate has expired (or is not valid), your app does not install or update on a device. Replace it with a renewed or valid certificate in your certificate profile, sign your app, and submit your app to Galaxy Store.

Author certificate
You do not need to connect your watch to your PC when generating an author certificate.

Distributor certificate
If you know the DUID of your watch, you do not need to connect your watch to your PC. You can manually add the DUID when you are generating the distributor certificate.

If you do not know the DUID of your watch, when you connect your watch to your PC, the DUID is automatically added when you are generating a distributor certificate. You can connect your watch over WiFi.

If your DUID starts with 1.0#, additional steps must be taken to be able to install your app on the device. See Permitting Device to Install Apps for more information.