I can’t sign my transaction

Metamask or other 3rd party apps might prevent you from spending your funds. This can happen when the (appropriate) private key is missing. 
 

Private keys, which are used for signing transactions, are derived from the combination of the seed stored on the Trezor device and a passphrase (if the passphrase feature is both enabled and used). 
 

Hence, if a different seed and/or a different passphrase is used then the private key differs, and so it cannot be used. In such a case, previously generated wallets/accounts in third-party apps would become watch-only wallets without the possibility of spending the funds.
 

To make sure you're using the correct passphrase, have a look at our videos:

Using passphrase in Trezor Suite, MetaMask and other apps

You can also check if your seed is correct with the 'Check backup' feature in Trezor Suite:

Test recovery seed on Trezor Model One

Once you initialize a MetaMask wallet, there is a default account created (the red one shown in the picture below). If you wish to use it, then you need to keep your MetaMask seed phrase to recover and access this account whenever needed. This account is not a part of your Trezor wallet.

On the contrary, there are Trezor accounts (the yellow and the blue one in the picture). The same accounts are generated in MEW, Trezor Suite etc. These are Trezor generated accounts and can be recovered only by using your Trezor recovery seed.

Even though Trezor is not connected, in MetaMask you can still see;

1. your default MetaMask account,
2. also, your previously used Trezor generated wallet(s) with different seed(s),
3. and also your Trezor generated wallet(s) with passphrase(s)

All three wallets mentioned (1, 2, 3) would differ from your currently used Trezor wallet. You can only use the first MetaMask account (1) without your Trezor because it’s completely independent of your Trezor.
 

However, as for the other accounts (2, 3), the related Trezor wallet must be connected (either with the appropriate seed and/or with the appropriate passphrase). Otherwise, you can't sign any transactions within these accounts due to missing the appropriate private key stored on your Trezor device, i.e., it gets frozen.