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First of all you will need to upgrade the Citrix license server to a supported version for 1912LTSR , Second i would check the SA/CSS eligibility date on your licenses as per https://support.citrix.com/article/CTX111618

After license server upgrade, I think you’ll have to generate again your license files (kinda license file update).

if CSS is expired, we wont be eligible for support from Citrix isnt it?

Yes

Thank you

check those burn in dates on your existing licenses for 1912 it has to be Dec 18, 2019 as listed from article i posted above. If it isn't as Steven mentioned you will have to regenerate your license file which should give you a new burn in date