The address was not accepted
A rejected address is a favour, not an obstacle. It is the final moment where a mistake costs nothing, because every chain writes addresses in its own shape and the check compares what you pasted against the shape the destination network uses.
You picked the wrong network for the address
This is most of the cases. Each chain has its own address alphabet and length, so an address from one network simply does not parse as an address on another. The check catches it because the two shapes cannot be confused, which is exactly why the same mistake is unrecoverable once a chain has accepted it instead.
Something came along with the paste
A trailing space, an invisible character from a chat message, a line break from a PDF, or a truncated string ending in an ellipsis will all fail the check. Retyping the last four characters by hand is the fastest way to find out, and comparing the first and last four against the source is the habit worth having.
It is a contract, not a wallet
Token contract addresses circulate in the same places wallet addresses do, and they look identical. Sending to one is a real transfer to a place that cannot send anything back. If you copied the address from a token page or a block explorer’s search box rather than from the person or service you are paying, check it again.
The network expects something extra
Some destinations, particularly exchange deposits, ask for a tag or a memo alongside the address. This wallet sends to an address and has no field for one, so a deposit that requires a tag needs a different route into that account. It is better to learn that here than after a transfer lands untagged.
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Several wallets in one account, unlinked on-chain
Hold as many wallets as you need inside one Telegram account. Their addresses are not linked on-chain, so paying from one tells nobody about the others. Send to a @username or to any address.
Questions
Why does the address work in another wallet?
Then check which network that wallet is sending on. The same string can be valid on one chain and meaningless on another, and the network selected here decides which shape is expected.
Can I send it anyway?
No, and that is deliberate. A transfer to a malformed address either fails on the chain or lands somewhere nobody controls, and neither outcome can be undone afterwards.
What if the recipient insists the address is right?
Ask which network they expect to receive on. Most of these disagreements are two people naming different chains for the same asset.
What if my exchange requires a memo?
Use a deposit method that does not, or deposit to an address at that exchange which needs no tag. There is nowhere here to attach one, and a missing tag is a support case at their end rather than a transfer that can be edited.