Hi @vito,
Welcome to the community 
Unfortunately I wasn’t able to reproduce your error.
I was able to use your token and migrations script to deploy to Truffle development network.
Can you try using the contract and migrations script and deploy to Truffle develop?
I am using WSL2 on Windows 10:
$ npx truffle version
Truffle v5.1.56 (core: 5.1.56)
Solidity - 0.6.12 (solc-js)
Node v10.22.1
Web3.js v1.2.9
I used the setup from: OpenZeppelin Upgrades: Step by Step Tutorial for Truffle
I installed OpenZeppelin Contracts Upgradeable
$ npm i @openzeppelin/contracts-upgradeable
Deploy
Using Truffle develop
$ npx truffle develop
Truffle Develop started at http://127.0.0.1:9545/
...
truffle(develop)> migrate
Compiling your contracts...
===========================
> Everything is up to date, there is nothing to compile.
Starting migrations...
======================
> Network name: 'develop'
> Network id: 5777
> Block gas limit: 6721975 (0x6691b7)
1_initial_migration.js
======================
Deploying 'Migrations'
----------------------
...
2_deploy_token.js
=================
Deploying 'MyToken'
-------------------
...
Deploying 'ProxyAdmin'
----------------------
...
Deploying 'AdminUpgradeabilityProxy'
------------------------------------
...
truffle(develop)> token = await MyToken.deployed()
undefined
truffle(develop)> await token.name()
'My Token'
truffle(develop)> (await token.totalSupply()).toString()
'100000000000000000000000'