There can be many possible causes.  For eamples, if you have more than one VPP tokens in your ABM such that some licenses have previously been reallocated/transferred improperly, or your MDM server have been upgraded with some old devices (with apps deployed)  improperly retired in the past, there might be discrepancies.  Maybe you can check if your problem is related to the above first.

Thank you for your message
It is probably defective and lost equipment.
Is there any way to correct the number ?

Have you deleted such defective and lost device(s) from your MobiControl server or unassigned  them in your Apple Business Manager portal?    There might be mismatch such that the defective/lost devices are still assigned on your ABM, and Apple backend probably won't be able to reclaim the deployed software licenses. 

If a defective device has been repaired and ready for re-deployment, you likely need to re-assign the device in ABM  if it has been unassigned, and then factory reset it to get it enrolled again.  The VPP license count should then be adjusted.

After your clean-up, I believe Apple's back-end should be able to update their database and reclaim the licenses originally associated with those defective and lost device(s), though I haven't thoroughly tested the flow.  Please let us know if you can solve the problem after the clean-up.

THANK YOU for the message

But one more question and that is the problem might also be fixed with version 15.3.2 with Resolved Issus

MCMR-24512 Apple app store licences (formerly known as VPP) could no longer be used after being revoked from Apple devices

????

For details on whether the issue solved for MCMR-24512 is related to your problem,  I think you can directly send email to Soti support team to confirm. 

If so, maybe you can consider whether it is worth the effort and/or risk(s) to upgrade to  MobiControl version 15.3.2. Upgrading to a new server version, especially for mission critical implementations or those with large number of enrolled devices,  should always be taken seriously, so that there won't be any bigger problem(s) newly introduced after the upgrade.

I might see this issue to simple..

The devices could installed the app already and are using the license. But the some of the devices
didn't synched with the MDM server to get the information that the app is installed. You could send
a check-in to your devices.

If there are devices who are not in use any longer and they got removed by you then you can use the
clean-up option. Just move to global settings to you VPP accounts. Select your VPP account and click
on clean-up. That will revoke all the VPP licenses from devices who are not enrolled to your MDM server.

Thank you for your message. I have carried out the instruction.
Unfortunately, the problem is still the same.

Have you confirmed that the lost/defective devices have been deleted in your MobiControl server and the same devices have been removed from the device list in your Apple Business Manager portal?  And have you sync the DEP/ADE token and cleaned up the VPP/ASL token subsequently?