Most research universities provide Mplus licenses on lab computers or via "Virtual Desktops" for students and faculty.
Mplus is a commercial statistical modeling program developed by Muthén & Muthén there is no official "free" full version of Mplus 6.12 Base Program and Combination Add-On Mplus 6.12 Base Program and Combination Add-on setup free
Would you like a step‑by‑step guide to setting up R with lavaan for the types of models Mplus 6.12 could run? Most research universities provide Mplus licenses on lab