Using an older version of the MELSOFT License Management tool with a newer version of GT Designer (or vice versa) causes compatibility breaks.
GT Designer3 has a 30‑day trial. After expiry → exactly this error. Solution: Purchase and install permanent license. gt designer license not found
Open the (if installed as part of GT Works). Using an older version of the MELSOFT License
Local configuration files that store licensing or environment paths have become corrupted. gt designer license not found
A license for GT Designer3 Version 1 does not work for GT Designer3 Version 2. Similarly, a license for GX Works3 (PLC programming) does not grant access to GT Designer3.