Can we put a lien on a commercial property , when we only have a contract signed by a tenant ??

It is possible that your mechanics lien could attach to both the leasehold and fee interests in the property. Of course, this may depend on whether a preliminary notice was properly and timely served, whether the owner was a "participating owner" in the tenant improvements and/or whether the owner timely and properly posted a notice of non-responsbility, and several other important factors. You should consult with an experienced construction attorney to discuss your particular issue.