Registration keys are typically version-specific. A key for DOSPRN 1.x will not work on DOSPRN 2.x.
Run DOSBox or vDos (a DOS emulator designed for printing). These tools allow you to set a static hardware ID, so once you register, the key never breaks.
Incorrect registry editing can harm your system. Back up your registry first (File > Export).
If you previously installed an older version of DOSPRN or a cracked/trial version, leftover registry keys can conflict with a new valid license. The software reads these corrupted remnants and declares the key invalid.
If you see a "Thank you" message but the registration window persists: