Error: Unsupported OpenTofu Core version

TL;DR: Your required_version constraint excludes the tofu binary you have installed. Either install a tofu version the constraint allows, or widen the constraint. This bites hardest when migrating from Terraform, because constraints like >= 1.14 follow Terraform's release line, which OpenTofu doesn't share.

Error: Unsupported OpenTofu Core version

  on versions.tf line 2, in terraform:
   2:   required_version = ">= 1.14"

This configuration does not support OpenTofu version 1.11.5.

Steps

1. Run tofu version and note the installed version. Expected: OpenTofu v1.11.5 (or whatever you have).

2. Decide: upgrade tofu, or relax the constraint. If the constraint references Terraform's line (for example >= 1.14 after a migration), edit versions.tf so it covers your tofu, e.g. required_version = ">= 1.11". Expected: the constraint now includes your installed version.

3. If you'd rather keep the constraint, install a matching tofu with a version manager: tofuenv install 1.12.6 && tofuenv use 1.12.6 (or the tenv equivalent). Expected: tofu version shows the new version.

4. Run tofu init. Expected: init proceeds past the version check.

When this applies

• tofu init fails immediately, naming a required_version line, right after migrating a repo from Terraform.
• CI installs a pinned tofu version that the repo's constraint doesn't allow.

When it doesn't apply

• Error: Failed to install provider is about provider versions, not the tofu binary.
• Error: Inconsistent dependency lock file is about provider pins, not the core version.

Tool versions

All OpenTofu versions. The check runs on every init, plan, and apply.

Why it happens

Version numbers diverged after the fork: Terraform and OpenTofu ship on separate cadences. A required_version written against Terraform's line (say >= 1.14) is meaningless on tofu's line (1.11.x, 1.12.x), so tofu refuses at init instead of running under a constraint it can't satisfy.

Edge cases

• During a transition where a repo must work with both tools, pick a floor both lines satisfy, like >= 1.6 (where OpenTofu forked), and tighten it once the migration is done.
• The error names the exact file and line, so you don't have to hunt; it's always the root module's terraform block.
• CI matrices should install the tofu version the repo declares. Pin it in one place (a .opentofu-version file read by tenv/tofuenv) so the number can't drift between local and CI.