(If it's just a permission error, the solution might be trivial.)
thanks for your response - I tried to update via Snap, i have very limited linux experience - this installed but then got the disk permissions errors when trying to run 0.20.1 ..... so I have gone back to 0.18
maybe both versions are conflicting or something
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If you Snap and you store the blockchain file on external storage, it's likely it's due to strict Snap permission, which doesn't allow application to access.
Check this thread
https://bitcointalk.org/index.php?topic=5211567.0, few people mention how to fix the problem.
I tried to download a tar file from bitcoin.org, it has files inside but no real guide as to how to install
You just need to extract tar file, then open directory
bin and double-click
bitcoin-qt or run this command
./bitcoin-qt
If there's execution permission problem, enable execution from ubuntu file manager or run this command
chmod +x bitcoin-qt